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figs. 17 and 18). They appear at 'first as outstanding processes on the surface of the body.

The exact mode in which the in-sinking of superficial outstanding limbs, carrying gill-lamellae, has historically taken place has been a matter of much speculation. It was to be hoped that the specimen of the Silurian scorpion (Palaeophonus) from Scotland, showing the ventral surface of the mesosoma (fig. 49), would throw light on this matter; but the specimen recently carefully studied by the writer and Pocock reveals neither gill-bearing limbs nor stigmata. The probability appears to be against an actual introversion of the appendage and its lamellae, as was at one time suggested by Lankester. It is probable that such an in-sinking as is shown in the

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FIG. 7.-Diagram of the dorsal surface of Limulus polyphemus.

oc, Lateral compound eyes.
oc', Central monomeniscous eyes.
PA, Post-anal spine.

I to VI, The six appendage-
bearing somites of the pro-

soma.

wise suppressed praegenital somite.

VIII to XIII, The six somites of the mesosoma, each with a movable pleural spine and a pair of dorsal entopophysis or VII, Usually considered to be muscle-attaching ingrowths. the tergum of the genital XIV to XVIII, The confluent or somite, but suggested by unexpressed six somites of the Pocock to be that of the other- metasoma.

[According to the system of numbering explained in the text, if VII is the tergum of the praegenital somite (as is probable) it should be labelled Prg without any number, and the somites VIII to XIII should be lettered 1 to 6, indicating that they are the six normal somites of the mesosoma; whilst XV to XVIII should be replaced by the numbers 7 to 12-an additional suppressed segment (making up the typical six) being reckoned to the metasomatic fusion.]

(From Lankester, Q. J. Micr. Sci. vol. xxi., 1881.) accompanying diagram has taken place (fig. 15); but we are yet in need of evidence as to the exact equivalence of margins, axis, &c., obtaining between the lung-book of Scorpio and the gill-book of Limulus. Zoologists are familiar with many instances (fishes, crustaceans) in which the protective walls of a water-breathing organ or gill-apparatus become converted into an air-breathing organ or lung, but there is no other case known of the conversion of gill processes themselves into air-breathing plates. The identification of the lung-books of Scorpio with the gill-books of Limulus is practically settled by the existence of the pectens in Scorpio (fig. 14, VIII) on the second mesosomatic somite. There is no doubt that these are parapodial or limb appendages, carrying numerous imbricated secondary processes, and therefore comparable in essential structure to the leaf-bearing plates of the second meso

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somatic somite of Limulus. They have remained unenclosed and projecting on the surface of the body, as once were the appendages of the four following somites. But they have lost their respiratory function. In non-aquatic life such an unprotected organ cannot subserve respiration. The "pectens have become more firmly chitinized and probably somewhat altered in shape as compared with their condition in the aquatic ancestral scorpions. Their present function in scorpions is not ascertained. They are not specially sensitive under ordinary conditions, and may be touched or even pinched without causing any discomfort to the scorpion. It is probable that they acquire special sensibility at the breeding season and serve as "guides" in copulation. The shape of the legs and the absence of paired terminal claws in the Silurian Palaeophonus (see figs. 48 and 49) as compared with living scorpions (see fig. 10) show that the early oc... scorpions were aquatic, and we may hope some day in better-preserved specimens than the two as yet discovered, to find the respira-c tory organs of those creatures in the condition of projecting appendages serving aquatic respiration somewhat as in Limulus, though not necessarily repeating the exact form of the broad plates of Limulus.

-It is important to note that the series of lamellae of the lung-book and the gill-book correspond exactly in structure, the narrow, flat blood-space in the lamellae being interrupted by pillar-like junctions of the two surfaces in both cases (see Lankester (4)), and the free surfaces of the adjacent lamellae being covered with a very delicate chitinous cuticle which is drawn out into delicate hairs and processes. The elongated axis which opens at the stigma in Scorpio and which can be cleared of soft, surrounding tissues and coagulated blood so as to present the appearance of a limb axis carrying the book-like leaves of the lung is not really, as it would seem to be at first sight, the limb axis. That is necessarily a blood-holding structure and is obliterated and fused with soft tissues of the sternal region so that the lamellae cannot be detached and presented as standing out from it. The apparent axis or basal support of the scorpion's lung-books shown in the figures, is a false or secondary axis and merely a part of the infolded surface which forms the air-chamber. The maceration of the soft parts of a scorpion preserved in weak spirit. and the cleaning of the chitinized in-grown cuticle give rise to the false appearance of a limb axis carrying the lamellae. The margins of the lamellae of the scorpion's lung-book, which are lowermost in the figures (fig. 15) and appear to be free, are really those which are attached to the blood-holding axis. The true free ends are those nearest the stigma.

XIV

-XV

XVI

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VII

LX

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XVII Anus

PA

FIG. 8. Diagram of the dorsal surface

Passing on now from the mesosoma we of a scorpion to comcome in Scorpio to the metasoma of six pare with fig. 7. segments, the first of which is broad whilst Letters and Roman the rest are cylindrical. The last is perforated numerals as in fig. 7, by the anus and carries the post-anal spine excepting that VII or sting. The somites of the metasoma carry is here certainly the no parapodia. In Limulus the metasoma is tergum of the first practically suppressed. In the allied extinct somite of the mesoEurypterines it is well developed, and re- soma -the genital sembles that of Scorpio. In the embryo somite-and 1S not Limulus (fig. 42) the six somites of the a survival of the emmesosoma are not fused to form a carapace bryonic praegenital at an early stage, and they are followed by somite. The anus (not three separately marked metasomatic somites; seen) is on the sternal the other three somites of the metasoma have surface. disappeared in Limulus, but are represented (From Lankester, loc. cit.) by the unsegmented prae-anal region. It is probable that we have in the metasoma of Limulus a case of the disappearance of once clearly demarcated somites. It would be possible to suppose, on the other hand, that new somites are only beginning to make their appearance here. The balance of various considerations is against the latter hypothesis. Following the metasoma in Limulus, we have as in Scorpio, the post-anal spine in this case not a sting, but a powerful and important organ of locomotion, serving to turn the animal over when it has fallen upon its back. The nature of the post-anal spine has been strangely misinterpreted by some writers. Owen (7) maintained that it represented a number of coalesced somites, regardless of its post-anal position and mode of development. The agreement of the grouping of the somites, of the form of the parapodia (appendages, limbs) in each region, of the position of the genital aperture and operculum, of the position and character of the eyes, and of the powerful post-anal spines not seen in other Arthropods, is very convincing as to the affinity

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of Limulus and Scorpio. Perhaps the most important general agreement of Scorpio compared with Limulus and the Eurypterines is the division of the body into the three regions (or tagmata)-prosoma, mesosoma and metasoma-each consisting of six segments, the prosoma having leg-like appendages, the mesosoma having foliaceous appendages, and the metasoma being destitute of appendages. In 1893, some years after the identification of the somites of Limulus with those of Scorpio, thus indicated, had been published, zoologists were startled by the discovery by a Japanese zoologist, Kishinouye (8), of a seventh prosomatic somite in the embryo of Limulus longispina. This was seen in longitudinal sections, as shown in fig. 19. The simple identification of somite with somite in Limulus and Scorpio seemed to be threatened by this discovery. But in 1896 Dr August Brauer of Marburg (9) discovered in the embryo of Scorpio a seventh prosomatic somite (see VII PrG, figs. 17 and 18), or, if we please so to term it, a praegenital somite, hitherto unrecognized. In the case of Scorpio this segment is indicated in the embryo by the presence of a pair of rudimentary appendages, carried by a well-marked somite. As in Limulus, so in Scorpio, this unexpected somite and its appendages disappear in the course of development. In fact, more or less complete "excalation" of the somite takes place. Owing to its position it is convenient to term the somite which is excalated in Limulus and Scorpio "the praegenital somite." It appears not improbable that the sternal plates wedged in between

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but is the ninth, owing to the presence or to the excalation of a praegenital somite. It seems that confusion and trouble will be best avoided by abstaining from the introduction of the non-evident somites, the ocular and the praegenital, into the numerical nomenclature of the component somites of the three great body regions. We shall, therefore, ignoring the ocular somite, speak of the first, second, third, fourth, fifth and sixth legbearing somites of the prosoma, and indicate the appendages by the Roman numerals, I, II, III, IV, V, VI, and whilst ignoring the praegenital somite we shall speak of the first, second, third, &c., somite of the mesosoma or opisthosoma (united mesosoma and metasoma) and indicate them by the Arabic numerals.

There are a number of other important points of structure besides those referring to the somites and appendages in which Limulus agrees with Scorpio or other Arachnida and differs from other Arthropoda. The chief of these are as follows:-.

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VIP

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FIG. 10.-Ventral view of a scorpion.

1. The Composition of the Palamnaeus indus, de Geer, to show Head (that is to say, of the the arrangement of the coxae of the anterior part of the pro- limbs, the sternal elements, genital soma) with especial Reference plate and pectens.

to the Region in Front of the M, Mouth behind the oval median Mouth. It appears (see

camerostome.

ARTHROPODA) that there is I, The chelicerae. embryological evidence of 11, The chelae.

the existence of two somites III to VI, the four pairs of walking legs. in Arachnida which were VIIgo, The genital somite or first originally post-oral, but have become prae-oral by adaptational shifting of the oral aperture.

somite of the mesosoma with the genital operculum (a fused pair of limbs). forwardly-slipped somites IXstg to XIIstg, the four pulmonary These VIIIP, The pectiniferous somite. are called "prosthomeres." The first of these has, in Arachnids met, The pentagonal metasternite of as in other the prosoma behind all the coxae. Arthropods, its pair of ap- x, The sternum of the pectiniferous pendages represented by the eyes. The second has

somites.

somite.

the last pair of legs in both Scorpio and Limulus, viz. the pentagonal for its pair of appendages 3, The broad first somite of the meta

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sternite of Scorpio (fig. 10) and the chilaria of Limulus (see figs. 13 and 20), may in part represent in the adult the sternum of the excalated praegenital somite. This has not been demonstrated by an actual following out of the development, but the position of these pieces and the fact that they are (in Limulus) supplied by an independent segmental nerve, favours the view that they may comprise the sternal area of the vanished praegenital somite. This interpretation, however, of the "metasternites" of Limulus and Scorpio is opposed by the coexistence in Thelyphonus (figs. 55, 57 and 58) of a similar metasternite with a complete praegenital somite. H. J. Hansen (10) has recognized that the "praegenital somite "persists in a rudimentary condition, forming a waist" to the series of somites in the Pedipalpi and Araneae. The present writer is of opinion that it will be found most convenient to treat this evanescent somite as something special, and not to attempt to reckon it to either the prosoma or the mesosoma. These will then remain as typically composed each of six appendage-bearing somites-the prosoma comprising in addition the ocular prosthomere. When the praegenital somite or traces of it are present it should not be called "the seventh prosomatic" or the first mesosomatic," but simply the "praegenital somite." The first segment of the mesosoma of Scorpio and Limulus thus remains the first segment, and can be identified as such throughout the Eu-arachnida, carrying as it always does the genital apertures. But it is necessary to remember, in the light of recent discoveries, that the sixth prosomatic pair of appendages is carried on the seventh somite of the whole series, there being two prosthomeres or somites in front of the mouth, the first carrying the eyes, the second the chelicerae; also that the first mesosomatic or genital somite is not the seventh or even the eighth of the whole series of somites which have been historically present, 1 See the article ARTHROPODA for the use of the term "prosthomere."

soma.

the small pair of limbs which in all living Arachnids is either chelate or retrovert (as in spiders), and is known as the chelicerae. It is possible, as maintained by some writers (Patten and others), that the lobes of the cerebral nervous mass in Arachnids indicate a larger number of prosthomeres as having fused in this region, but there is no embryological evidence at present which justifies us in assuming the existence in Arachnids of more than two prosthomeres. The position of the chelicerae of Limulus and of the ganglionic nerve-masses from which they receive their nerve-supply, is

(From a drawing by Pocock.)

closely similar to that of FIG. 11-Third leg of Limulus polythe same structures in phemus, showing the division of the fourth Scorpio. The cerebral segment of the leg by a groove S into mass is in Limulus more two, thus giving seven segments to the easily separated by dis- leg as in scorpion. section as a median lobe distinct from the laterallyplaced ganglia of the cheliceral somite than is the case in Scorpio, but the relations are practically the same in the two forms. Formerly it was supposed that in Limulus both the chelicerae and the next following pair of appendages were prosthomerous, as in Crustacea, but the dissections of Alphonse Milne-Edwards (6) demonstrated

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palpiform appendages. But although in such lower Crustacea the nerve-ganglia of the third prosthomere have not fused with the anterior nerve-mass, there is no question as to the prae-oral position of two appendage-bearing somites in addition to the ocular prosthomere. The Crustacea have, in fact, three prosthomeres in the head and the Arachnida only two, and Limulus agrees with the Arachnida in this respect and differs from the Crustacea. The central nervous systems of Limulus and of Scorpio present closer agreement in structure than can be found when a Crustacean is compared with either. The wide divarication of the lateral cords in the prosoma and their connexion by transverse commissures, together with the "attraction" of ganglia to the prosomatic ganglion group which

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properly belong to hinder segments, are very nearly identical in the two animals. The form and disposition of the ganglion cells are also peculiar and closely similar in the two. (See Patten (42) for important observations on the neuromeres, &c., of Limulus and Scorpio.) 2. The Minute Structure of the Central Eyes and of the Lateral Eyes.-Limulus agrees with Scorpio not only in having a pair of central eyes and also lateral eyes, but in the microscopic structure of those organs, which differs in the central and lateral eyes respectively. The central eyes are "simple eyes," that is to say, have a single lens, and are hence called "monomeniscous." The lateral eyes are in Limulus "compound eyes," that is to say, consist of many lenses placed close together; beneath each lens is a complex of protoplasmic cells, in which the optic nerve terminates. Each such unit is termed an ommatidium.' The lateral eyes of Scorpio consist of groups of separate small lenses each with its ommatidium, but they do not form a continuous compound eye as in Limulus. The ommatidium (soft structure beneath the lens-unit of a compound eye) is very simple in both Scorpio and Limulus. It consists of a single layer of cells, continuous with those which secrete the general chitinous 'covering of the prosoma. The cells of the ommatidium are a good deal larger than the neighbouring common cells of the epidermis. They secrete the knob-like lens (fig. 22). But they also receive the nerve fibres of the optic nerve. They are at the same time both optic nerve-end cells, that is to say, retina cells, and corneagen cells or secretors of the chitinous lens-like cornea. In Limulus (fig. 23) each ommatidium has a peculiar ganglion cell developed in a central position, whilst the ommatidium of the lateral eyelets of Scorpio shows small intermediate cells between the larger nerve - end cells. The structure of the latéral eye of Limulus was first described by Grenacher, and further and more accurately by Lankester and Bourne (5) and by Watase; that of Scorpio by Lan

st

st

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ja

kester and Bourne, FIG. 13.-Diagrams of the meta-sternite st, who showed that with genital operculum op, and the first lamellithe statements of gerous pair of appendages ga, with uniting von Graber were sternal element st of Scorpio (left) and Limulus erroneous, and (right).

that the lateral

eyes of Scorpio

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(From Lankester, loc. cit.)

have a single cell-layered or "monostichous" ommatidium like that of Limulus. Watase has shown, in a very convincing way, how by deepening the pit-like set of cells beneath a simple lens the more complex ommatidia of the compound eyes of Crustacea and Hexapoda may be derived from such a condition as that presented in the lateral eyes of Limulus and Scorpio. (For details the reader is referred to Watase (11) and to Lankester and Bourne (5).) The structure of the central eyes of Scorpio and spiders and also of Limulus differs essentially from that of the lateral eyes in having two layers of cells (hence called diplostichous) beneath the lens, separated from one another by a membrane (figs. 24 and 25). The upper layer is the corneagen and secretes the lens, the lower is the retinal layer. The mass of soft cell-structures beneath a large lens of a central eye is called an ommatoeum." It shows in Scorpio and Limulus a tendency to segregate into minor groups or "ommatidia." It is found that in embryological growth the retinal layer of the central eyes forms as a separate pouch, which is pushed in laterally beneath the corneagen layer from the epidermic cell layer. Hence it is in origin double, and consists of a true retinal layer and a post-retinal layer (fig. 24, B), though these are not separated by a membrane. Accordingly the diplostichous ommatoeum or soft tissue of the Arachnid's central eye should strictly be called triplostichous," since the deep layer is itself doubled or folded. The retinal cells of both the lateral and central eyes of Limulus and Scorpio produce cuticular structures on their sides; each such piece is a rhabdomere and a number (five or ten) uniting form a rhabdom (fig. 26). In the specialized ommatidia of the compound eyes of Crustacea and Hexapods the rhabdom is an important structure. It is a very significant fact that the lateral and central eyes of Limulus and Scorpio not only agree each with each in regard to their monostichous and diplostichous structure, but also in the formation in both classes of eyes of rhabdomeres and rhabdoms in which the component pieces are five or a multiple of five (fig. 26). Whilst each unit of the lateral eye of Limulus has a rhabdom of ten pieces 1 See fig. 12 in the article ARTHROPODA.

2 Though ten is the prevailing number of retinula cells and rhabdomeres in the lateral eye of Limulus, Watase states that they may be as few as nine and as many as eighteen.

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opening remains in the adult scorpion. In all the embryonic or permanent opening is on the coxa of the fifth pair of prosomatic limbs. Thus an organ newly discovered in Scorpio was found to have its counterpart in Limulus.

"

The name coxal gland" needs to be carefully distinguished from "crural gland," with which it is apt to be confused. The crural glands, which occur in many terrestrial Arthropods, are epidermal in origin and totally distinct from the coxal glands. The coxal glands of the Arachnida are structures of the same nature as the green glands of the higher Crustacea and the so-called shell glands" of the Entomostraca. The latter open at the base of the fifth pair of limbs of the Crustacean, just as the coxal glands open on the coxal joint of the fifth pair of limbs of the Arachnid. Both belong to the category of "coelomoducts," namely, tubular or funnel-like portions of the coelom opening to the exterior in pairs in each somite (potentially,) and usually persisting in only a few somites as either "urocoels" (renal organs) or "gonocoels" (genital tubes). In Peripatus they occur in every somite of the body. They have till recently been very generally identified with the nephridia of Chaetopod worms, but there is good reason for considering the true nephridia (typified by the nephridia of the earthworm) as a distinct class of organs (see Lankester in vol. ii. chap. iii. of A Treatise on Zoology, 1900). The genital ducts of Arthropoda are, like the green glands, shell glands and coxal glands, to be regarded as coelomoducts (gonocoels). The coxal glands do not establish any special connexion between Limulus and Scorpio, since thay also occur in the same somite in the lower Crustacea, but it is to be noted that the coxal glands of Limulus are in minute structure and probably in function more like those of Arachnids than those of Crustacea.

FIG. 14. The first three pairs of mesosomatic appendages of Scorpio and
Limulus compared.

VII, The genital operculum.
VIII, The pectens of Scorpio and the
first branchial plate of Limulus.
IX, The first pair of lung-books of
Scorpio and the second branchial
plate of Limulus.

gp, Genital pore.

epst, Epistigmatic sclerite.
stg, Stigma or orifice of the hollow
tendons of the branchial plates of
Limulus.

(After Lankester, loc. cit.)

eyes, it is to be noted that no Crustacean has structures corresponding to the peculiar diplostichous central eyes, though these occur again (with differences in detail) in Hexapoda. Possibly, however, an investigation of the development of the median eyes of some Crustacea (Apus, Palaemon) may prove them to be diplostichous in origin.

3. The so-called "Coxal Glands."-In 1882 (Proc. Roy. Soc. No. 221) Lankester described under the name "coxal glands" a pair of brilliantly white oviform bodies lying in the Scorpion's prosoma immediately above the coxae of the fifth and sixth pairs of legs (fig. 27). These bodies had been erroneously supposed by Newport (12) and other observers to be glandular outgrowths of the alimentary canal. They are really excretory glands, and communicate with the exterior by a very minute aperture on the posterior face of the coxa of the fifth limb on each side. When examined with the microscope, by means of the usual section method, they are seen to consist of a labyrinthine tube lined with peculiar cells, each cell having a deep vertically striated border on the surface farthest from the lumen, as is seen in the cells of some renal organs. The coils and branches of the tube are packed by connective tissue and blood spaces. A similar pair of coxal glands, lobate instead of ovoid in shape, was described by Lankester in Mygale, and it was also shown by him that the structures in Limulus called "brick-red glands" by Packard have the same structure and position as the coxal glands of Scorpio and Mygale. In Limulus these organs consist each of four horizontal lobes lying on the coxal margin of the second, third, fourth, and fifth prosomatic limbs, the four lobes being connected to one another by a transverse piece or stem (fig. 28). Microscopically their structure is the same in essentials as that of the coxal glands of Scorpio (13). Coxal glands have since been recognized and described in other Arachnida. In 1900 it was shown that the coxal gland of Limulus is provided with a very delicate thin-walled coiled duct which opens, even in the adult condition, by a minute pore on the coxa of the fifth leg (Patten and Hazen, 13A). Previously to this, Lankester's pupil Gulland had shown (1885) that in the embryo the coxal gland is a comparatively simple tube, which opens to the exterior in this position and by its other extremity into a coelomic space. Similar observations were made by Laurie (17) in Lankester's laboratory (1890) with regard to the early condition of the coxal gland of Scorpio, and by Bertkau (41) as to that of the spider Atypus. H. M. Bernard (138) showed that the

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4. The Entosternites and their Minute Structure-StraussDürckheim (1) was the first to insist on the affinity between Limulus and the Arachnids, indicated by the presence of a free suspended entosternum or plastron or entosternite in both. We have figured here (figs. I to 6) the entosternites of Limulus, Scorpio and Mygale. Lankester some years ago made a special study of the histology (3) of these entosternites for the purpose of comparison, and also ascertained the relations of the very numerous muscles which are inserted into them (4). The entosternites are cartilaginous in texture, but they have neither the chemical character nor the microscopic structure of the hyaline cartilage of Vertebrates. They yield chitin in place of chondrin or gelatin-as does also the cartilage of the Cephalopod's endoskeleton. In microscopic structure they all present the closest agreement with one another. We find a firm, homogeneous or sparsely fibrillated matrix in which are embedded

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FIG. 15.-The remaining three pairs of mesosomatic appendages of Scorpio and Limulus. Letters as in fig. 14. 130 indicates that there are 130 lamellae in the scorpion's lung-book, whilst 1150 indicates that 150 similar lamellae are counted in the gill of Limulus.

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FIG. 17.-Embryo of scorpion, ventral view showing somites and appendages.

sgc, Frontal groove.

mesosoma is a small, free entosternite having a similar position, but below or ventral to the nerve cords, and having a smaller number of muscles attached to it. The entoster

nite was probably in origin part of the fibrous connective tissue lying close to the integument of the sternal surfacegiving attachment to muscles corresponding more or less to those at present attached to it. It became isolated and

detached, why or with what advantage to the organism it is difficult to say, and at that period of Arachnidan development the great ventral nerve cords occupied a more lateral position than they do at sa, Rudiment of lateral eyes. present. We know that such obl, Camerostome (upper lip). a lateral position of the nerve so, Sense-organ of Patten. cords preceded the median PrGabp, Rudiment of the appen- position in both Arthropoda dage of the praegenital somite and Chaetopoda. Subsewhich disappears. quently to the floating off of abp, Rudiment of the right half of the entosternite the approxithe genital operculum. abp3, Rudiment of the right pecten. abp to abp', Rudiments of the four appendages which carry the pulmonary lamellae.

I to VI, Rudiments of the six limbs
of the prosoma.
VIIPrG, The evanescent praegenital
somite.

mation of the nerve cords took place in the prosoma, and thus they were able to take up a position below the entosternite. In the mesosoma the approximation had occurred before the entosternites were formed.

which connect it with the

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entosternite gives rise to outgrowths, besides the great posterior flaps, pf, which form the diaphragm, unrepresented in Limulus. These are a ventral arch forming a neural canal through which the great nerve cords pass (figs. 3 and 4, snp), and further a dorsal gastric canal and arterial canal which transmit the alimentary tract and the dorsal artery respectively (figs. 3 and 4, GC, DR).

VIIPIG go

In Limulus small entosternites are found in each somite of the appendage-bearing mesosoma, and we the mesosoma which has a wellfind in Scorpio, in the only somite of developed pair of appendages, that of the pectens, a small entosternite with ten pairs of muscles inserted into it. The supra-pectinal entosternite lies ventral to the nerve cords.

In Mygale (figs. 5 and 6) the form of the entosternite is more like that of Limulus than is that of Scorpio. The anterior notch Ph.N. is similar to that in Limulus, whilst the imbri

VIII

Km

IX

.abp abp5

ab pe

abp

cate triangular pieces of the posterior FIG. 18.-Portion of a simimedian region resemble the similarly- lar embryo at a later stage placed structures of Limulus in a of growth. The praegenital striking manner. somite, VII PrG, is still It must be confessed that we are present, but has lost its singularly ignorant as to the functional rudimentary appendages; significance of these remarkable organs go, the genital operculum, -the entosternites. Their movement left half; Km, the left in an upward or downward direction pecten; abp to abp, the in Limulus and Mygale must exert a rudimentary appendages of pumping action on the blood con- the lung-sacs. tained in the dorsal arteries and the

ventral veins respectively. In Scorpio

(After Brauer, loc. cit.)

the completion of the horizontal plate by oblique flaps, so as to form an actual diaphragm shutting off the cavity of the prosoma from the rest of the body, possibly gives to the organs contained in the anterior chamber a physiological advantage in respect of the supply of arterial blood and its separation from the venous blood of the mesosoma. Possibly the movement of the diaphragm may determine the passage of air into or out of the lung-sacs. Muscular fibres connected with the suctorial

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pharynx are in Limulus inserted into the entosternite, and the activity of the two organs may be

correlated.

op capp 5. The Blood and the BloodFIG. 19.-Section through an vascular System.-The blood fluids early embryo of Limulus longiof Limulus and Scorpio are very spina, showing seven transverse similar. Not only are the blood divisions in the region of the corpuscles of Limulus more like unsegmented anterior carapace. in form and granulation to those The seventh, VII, is anterior to the genital operculum, op, and of Scorpio than to those of any Crustacean, but the fluid is in is the cavity of the praegenital both animals strongly impreg- somite which is more or less nated with the blue-coloured completely suppressed in subrespiratory proteid, haemocyanin, sequent development, possibly This body occurs also in the blood indicated by the area marked of Crustacea and of Molluscs, but VII in fig. 7 and by the great its abundance in both Limulus entopophyses of the prosomatic and Scorpio is very marked, and carapace. gives to the freshly-shed blood a strong indigo-blue tint. The great dorsal contractile vessel or heart" of Limulus is closely similar to that of Scorpio; its ostia or incurrent orifices are

Ch

In the scorpion (figs. 3 and 4) the entosternite has tough VIII, The first mesosomatic somite membrane - like outgrowths or genital somite. IX, The second mesosomatic somite body-wall, both dorsally and or pectiniferous somite. ventrally forming an oblique pmst X to XIII, The four pulmoniferous diaphragm, cutting off the somites. cavity of the prosoma from XIV, The first metasomatic somite. that of the mesosoma. It was (After Brauer, Zeilsch. wiss. Zool. vol. lix., described by Newport as "the 1895.) diaphragm. Only the central and horizontal parts of this structure correspond precisely to the entosternite of Limulus: the right and left anterior processes (marked ap in figs. 3 and 4, and RAP, LAP, in figs. 1 and 2) correspond in the two animals, and the median lateral process Imp of the scorpion represents the tendinous outgrowths ALR, PLR of Limulus. The scorpion's

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placed in the same somites as those of Scorpio, but there is one additional posterior pair. The origin of the paired arteries from the

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