« السابقةمتابعة »
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 ‘cotland, 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
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 lun -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-a paratus become converted into an air—breathing organ or lung. ut there is no other case known of the conversion of ill processes themselves into air-breathing plates.
he identification of the lung-books of Scorpio with the gill-books of Limulus is practically settled by the existence of the pectens in Sco ‘ io (fig. I4, VIII) on the second mesosomatic somite. There is no oubt that these are parapodial or limb appendages, carrying numerous imbricated secondary processes, and therefore com rable in essential structure to the leaf-bearing plates of the secon meso
VIII to XIII, The six somites of the mesosoma, each with a movable pleural spine and a pair of dorsal entopophysis or muscle-attachin ingrowths.
XIV to XVIII, T e confluent 0r unexpressed six somites of the metasoma.
somatic somite of Limulus. They have remained unenclosed and projecting on the surface of the body, as once were the appendages oi 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 com red with their condition in the aquatic ancestral sco ions. heir present function in scorpions is not ascertained. ey 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 Palaeaphonus (see figs. 48 and 49) as compared with living scorpions (see fig. 10) show that the early on. ' scorpions were aquatic, and we may hope some day in better-preserved specimens than the two as yet discovered, to find the respira— tory organs of those creatures in the condition of projecting appendages serving aquatic respiration somewhat as in Limulus, though not necessarily re ating the exact form of the broad plates 0 Limulus.
It is im rtant to note that the series of lamellae o the lung-book and the gill-book correspond exactly in structure, the narrow, fiat blood-space in the lamellae bein interrupted by pillar-like junctions of t e two surfaces in both cases (see Lankcster (4)). and the free surfaces ofthe 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 sti ma in Scorpio and which can be cleared 0 soft, surrounding tissues and coa ulated blood so as to present the appearance 0% a limb axis carrying the book-like leaves ' of the lung is not rcall , as it would seem to be at first sight, the lim axis. That is necessarin 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 a parent axis or basal support of the scor ions lung-books shown in the figures, is a also or secondary axis and merely a part of the infolded surface which forms the air-chamber. The macerationpf 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 lun -book, which are lowermost in the fi ures ( g. 15) and appear to be free, are real y those which are attached to the blood-holdin axis. The true free ends are those nearest t e stigma.
Passing on now from the mesosoma we come in Scorpio to the metasoma of six segments, the first of which is broad whilst the rest are cylindrical. The last is perforated by the anus and carries the post-anal spine or sting. The somites of the metasoma carry no parapodia. In Limulus the metasoma is practically suppressed. In the allied extinct Eur terines it is well developed, and resemblls thatof Scorpio. In the embr o Limulus (fig. 42) the six somites of the mesosoma are not fused to form a carapace at an early stage, and they are followed by three separately marked metasomatic somites; the other three somites of the metasoma have disappeared in Limulus, but are represented 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 groupin ofthe somites, of the form of the parapodia (appendages, limbs) in eac region, of the position of the genital aperture and o erculum. of the position and characterof the eyes, and of the powerfu post-anal spines not seen in other Arthropods, is very convincing as to the afiinity
FIG. 8.—Diagrarn of the dorsal surface of a scorpi0n to corn
are with fig. 7.
tters and Roman numerals as in fig. 7, excepting that VII is here certainly the tergum of the first somite of the mesosoma—the genital somite—~and IS not a survival of the embryonic praegenital somite. The anus (not seen) is on the sternal surface.
(From Lankrsler, lot. (17.)
of Limulus and Scorpio. Perhaps the most im rtant'general agreement of Scorpio compared with imulus and tin): Eurypterines is the division of the body into the three regions (or tagmata)—prosoma, mesosoma and metasoma—each consisting of six segments, the pmsoma having le ~like appendages, the mesosoma havin foliaceous ap ndages, and t e metasoma being destitute of appen ages.
l: 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 emb o of Limuluslon ispina. This wasseen in longitudinal sections, ass own in fig. I9. he simple identification of somite with somite in Limulus and Scorpio seemed to be threatened by this discovery. But in 18%: Dr August Brauer of Marburg (9) discovered in the embryo of orpio a seventh prosomatic somite (see VII PrG, figs. 17 and 18), or, if we please so to term it, a Praegem'tal somite, hitherto unrecognized. In the case of Scorpio this segment is indicated in the embryo by the presence of a pair of rudimentary a pendages, carried by a well-marked somite. As in Limulus, so in orpio, 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 sition it is convenient to term the somite which is excalated in imulus and Scorpio " the praegenital somite." It appears not improbable that the sternal p ates wedged in between
the last pair of legs in both Scorpio and Limulus, viz. the pentagonal sternite of Scorpio (fig. IO) 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 ma comprise the sternal area of the vanished praegenital somite. his 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. . J. Hansen (10) has recognized that the “ praegenital somite " persists in a rudimenta condition. forming a “waist” to the series of somitcs in the lgedipalpi 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 mch of six appendage-bearing somites—the prosoma comprising in addition the ocular prosthomere.l When the praegenital somite or traces of it are resent it should not be called “the seventh prosomatic " or the ' first mesosomatic," but simply the “ praegemtal 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 rosomatic pair of appendages is carried on the seventh somite ofp 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 enital somite is not the seventh or even the eighth of the whole series of somites which have been historically present,
but is the ninth, owing to the resenoe or to the excalation of a raegenitalv somite. It seems t at confusion and trouble will be best avoided by abstainin from the introduction 0 the non-evident somitcs, the ocular and the praegenital, into the numerical nomenclature of the component somites of the three great body regions. We shall, therefore, ignorin the ocular somite, speak 0 the first, second, third, fourth, fifth and sixth leg~ bearing somites of the prosoma,d and bindtilcatltz the a nages yt e oman ngi'heerals, 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 opistho‘ soma (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 :—.
t. The Composition 0 the Head (that is to say, 0 the anterior art of the prosoma) wit especial Reference to the Region in Front of the Mouth—It ap ars (see ARTHROPODA) t at there is embryological. evidence of the existence of two somites in Arachnida which were originally post-oral, but have become prae-oral by adaptational shifting of the
Hill FIG. 10.—Ventral view of a scorpion. Palamnaeus indus, de Geer, to show the arrangement of the coxae of the limbs, the stei'nal elements, genital plate and pectens. M, Mouth behind the oval median camerostome. I, The chelicerae. II, The chelae. III to VI, the four pairs of walking legs. VlIgo, The enital somite or first somite of t e mesosoma with the enital operculum (a fused pair of
met, The pentagonal metasternite of the prosoma behind all the coxae. x, The sternum of the pectiniferous
Arachnids as in other Arthropods, its pair of ap
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 Arachuids indicate a larger number of rosthomeres as having used in this region, but there is no embryological evidence at present which justifies us in assumin the existence in Arachni s of more than two rosthomeres. The position of the chelicerae of Limulus and of the ganglionic nerve-masses mm which they receive their nerve-supp! , is closely similar to t at of the same structures in Scorpio. The cerebral mass is in Limulus more easily separated by dissection as a median lobe distinct from thelaterallyplaced ganglia of the cheliceral somite than is the case in Scorpio, but the relations are practicall the same in the two forms. Formerly it was supposed that in Limulus both the chelicerae and the next following pair of appendages were rosthomerous, as in Crustacea, but the dissections of Alphonse ilne‘Edwards (6) demonstrated
FIG. IL—Third leg of Limulus Polyphemus, showing the division of the fourth segment of the leg by a groove S into two, thus giving seven segments to the leg as in scorpion.
(From a drawing by Pocock.)
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 ap ndage-bearing somites in addition to the ocular prosthomere. T e Crustacea have, in fact, three prosthomeres in the head and the Arachnida onl two, and Limulus agrees with the Arachnida in this respect and diffia'rs from the Crustacea. The central nervous systems of Limulus and of Scorpio resent closer agreement in structure than can be found when a €rustacean 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
’covering of the prosoma.
properly belon to hinder segments. are very nearly identical in , the two animals. he form and disposition of the gan lion 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 a rees with Scorpio not onl in having a pair of central eyes and a so 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 smgle 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 The cells of the ommatidium are a good deal larger than the neighbouring common cells of the epidermis. They secrete the knob-like lens (fi . 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. ln 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 lar er nerve - end cel s. The structure of the lateral 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 Lankester and Bourne, who showed that the statements of von Graber were erroneous, and that the lateral eyes of Scor i0 have a single ce l-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 com
lex ommatidia of the compound eyes of C rustaceaand Hexa oda may e derived from such a condition as that resented in t e lateral eyes of Limulus and Scorpio. (For detai s 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 e es in having two layers of cells (hence called diplostichous) beneat 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 rowth the retinal layer of the central egos forms as a separate pouc , which is pushed in laterally beneath t e 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. Accordin ly the diplostichous ommatoeum or soft tissue of the Arachnid 5 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 Hexa ds the rhabdom is an important structure.l his a very signifigmt 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 arhabdom of ten2 pieces
1 See fig. 12 in the article ARTHROPODA.
1 Though ten is the prevailing number of retinula cells and rhabd0~ meres in the lateral eye of Limulus, Watase states that they may be as few as nine and as many as eighteen. gm 3° Jug,
gerous pair of ap ndages go, with uniting sternal element st 0 Scorpio (left) and Limulus (right). >
(From Lanltesler, lac. cit.)
forming a star-like chitinous centre in section, each lateral eye of Scorpio has several rhabdoms of five or less rhabdomeres, indicating that the Limulus lateral eye-unit is more specialized than the detached lateral eyelet of Scorpio, so as to present a coincidence of one lens with one rhabdom. Numerous rhabdomeres (grouped as rhabdoms in Limulus) are found in the retinal layer of the central eyes also. Whilst Limulus agrees thus closely with Scorpio in regard to the
eyes, it is to be noted that no Crustacean has structures corresponding to the Eeculiar diplostichous central eyes, though these occur again (wit difiemnces 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 G!ands."—ln 1882 (Proc. Roy. Sac. No. 22!) 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 ovuid 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. ln 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 osition and by its other extremity into a coelomic space. Similar o servations were made h Laurie (17) in Lankester's laborato (1890) with re ard to t e early condition of the coxal gland o Scor i0, and by ertkau (41) as to that of the spider Atypus. H. M. B
stg, Stigma or orifice of the hollow' tendons of the branchial plates of
crnard (138) showed that the ,
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 hi her Crustacea and the so-called “shell glands " of the ntomostraca. The latter open at the base of the fifth pair of limbs of the Crustacean, just as the coxal glands open on the coxal 'oint of the fifth pair of limbs of the Arachnid. Both
elong 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" (renalorgans)or"gonocoels"( enital tubes). In Peripatus they occur in every somite o the body. They have till recently been very generally identified with the nephridia of Chaeto od worms, but there is good reason for considering t e 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).v The genital ducts of Arthropoda are, like the green glands, shell glands and coxal lands, 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.
4. The Entostemiles and their Minute Shudurm—Strauss Diirckheim (l) was the first to insist on the affinity between Limulus and the Arachnids, indicated by the presence of a free suspended entosternum or lastron 0r entosternite in both. We have figured here ( gs. 1 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 microsco ic structure of the~hyaline cartilage of Vertebrates. They yied chitin in place of chondrin or gelatin—as does also the cartilage of the Cephalopod's endoskeleton. ln micr0scopic structure they all Eresent the closest agreement with one another. We find a firm, omogeneous or sparsely fibrillated matrix in which are embedded
Letters as In fig. 14. 1130 indicates that there are 130 lame] ae
nucleated cells (corpuscles of protoplasm) arran ed in rows of three. six or eight, parallel with the adjacent lines of brillation.
A minute entosternite having the above-described structure is found in the Crustacean Apus between the bases of the mandibles, and also in the Decapoda in a similar position, but in no Crustacean does it attain to any size or importance. On the other hand. the entosternite of the Arachnida is a very large and important feature
to it from the bases of the surrounding limbs and from the dorsal carapace and from the pharynx. It consists of an oblong plate 2 in. in length and i in breadth, with a pair of tendmous outgrowths standing out from it at right angles on each side. It " floats "
entosternite gives rise to outgrowths, besides the great sterior flaps,v pf, which form the diaphragm, unrepresented in imulus. These are a ventral arch forming a neural canal through which the great nerve cords pass (figs. 3 and 4, rap), and further a dorsal gastric canal and arterial canal which transmit the alimentary tract and the dorsal artery respectively (figs. 3 and 4, CC, DR). .
In Limulus small entosternites are found in each somite of the appendage-bearing mesosoma. and we find in Scorpio, in the only somite of the mesosoma which has a welldcveloped pair of a pendages, that of the pectens, a smalFentosternite 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 cntosternite is more like that of Limulus than is that of Scorpio. The anterior notch Ph.N. is similar to that in Limulus, whilst the imbricate triangular pieces of the sterior median region resemble the similarlyplaced structures of Limulus in a striking manner.
It must be confessed that we are singularly ignorant as to the functional significance of these remarkable organs —the entosternites. Their movement in an upward or downward direction in Limulus and Mygale must exert a pumping action on the blood contained in the dorsal arteries and the ventral veins respectively. In Scorpio 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
FIG. i8.—Portion ofa similar embryo at a later stage of growth. The raegenital somite, VII Prg, is still present, but has lost its rudimentary appendages; go, the enital operculum, left hal; Km, the left pecten; abp‘ to ob 7, the rudimentary appen ages of the lung-sacs.
(After Brauer, lac. cit.)
between the prosomatic nerve centres and the alimentary canal. In each somite of the 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 entosternite was probably in origin part of the fibrous connective tissue lyin close to the integument of the sternal surface— giving 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 present. We know that such a lateral position of the nerve cords preceded the median position in both Arthropoda and Chaetopoda. Subsequently to the floating off of the entosternite the approximation of the nerve cords took place in the prosoma, and thus they were able to take up a osition below the entosternite. n the mesosoma the a proximation had occurred be ore the entOsternites were formed.
In the scorpion (figs. 3 and 4) the entosternite has tou h membrane-like outgrowt s which connect it With the body-wall, both dorsall and ventrally forming an oblique diaphragm, cutting off the cavity of the prosoma from that of the mesosoma. It was described by New ort as “ the diaphragm. ’ On y the central
anterior chamber a physiological advantage in res ct of the supply of arterial bloo 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
harynx are in Limulus inserted into the entosternite, and the activity of the two organs may be correlated.
5. The Blood and the Bloodvascular System.—~The blood fluids of Limulus and Seorpio are very similar. Not only are the blood cor uscles of Limulus more like in orm and granulation to those of Scorpio than to those of any Crustacean, but the fiuid~is in both animals stron ly impregnated with the b ue-coloured respiratory proteid, haemocyanin. Thisbody occurs also in the blood of Crustacea and of Molluscs, but its abundance in both Limulus and Scorpio is very marked, and 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;
and horizontal parts of this structure correspond precisely to the entosternite of Limulus: the right and left anterior processes(marked up in figs. 3 and 4, and RAP, LAP, in figs. I and 2) correspond in the two animals, and the median lateral process lmp of the scor ion represents the tendinous outgrowths ALR, PLR of Limulus. PThe scorpion's
size. (After Lankester.)
placed in the some somites as those of Scorpio, but there is one additional posterior pair. The origin of the paired arteries from the