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artery in close relation to the ventral nerve cords—has been described in several cases. On the other hand, in many Arthropods, especially those which possess tracheae. the arteries do not have a long course, but soon open into wide blood sinuses. Scorpio certainly comes nearer to Limulus in the high development of its arterial system, and the intimate relation of the anterior aorta and its branches to the nerve centres and great nerves, than does any other Arthropod.
An arrangement of great functional importance in regard to the venous system must now be described, which was shown in [883 by Lankester to be common to Limulus and Scorpio. This arrangement has not hitherto been detected in any other class than the Arachnida. and if it should ultimately prove to be eculiar to that grou , would have considerable weight as a proof oi) the close genetic a nity of Limulus and Scorpio.
FIG. 24.—Dia rams of the development and adult structure of one of the paired central eyes of a scorpion.
A, Early condition before the lens is deposited, showing the folding of the epidermic cell-layer into three.
B, Diagram showing the nature of this infolding.
C, Section through the fully formed eye.
h, Epidermic cell-layer. _
r, The retinal portion of the same which, owing to the infolding, lies between gl, the corneagen or lens-forming portion, and pr, the
ost-retinal 0r capsular portion or fold.
l. uticular lens.
g, Line separating lens from the lens-forming 0r corneagen cells of theepidermis.
n, Nerve fibres.
rh, Rhabdomeres. [How the inversion of the nerve-end—cells and their connexion with
the nerve-fibres is to be reconciled with the condition found in the
adult, or with that of the monostichous eye, has not hitherto been
ex lained.] p (F rorn Korschelt and Heider.)
The eat ricardial sinus is strongly developed in both animals. lts walfsfare brous and complete, and it holds a considerable volume of blood when the heart itself is contracted. Opening in pairs in each somite, right and left into the pericardial sinus are lar e veins, which bring the blood respectively from the gill-books and t e lungbooks to that chamber. whence it passes by the ostia into the heart. The blood is brought to the respiratory organs in both cases by a
rent venous collecting sinus having a ventral median position. in both animals the wall of the Pericardial sinus is connected by vertical muscular bands to the wall of the ventral venous sinus (its lateral expansions around the lung-books in Scorpio) in each somite through which the pericardium passes. There are seven pairs ofthese venogericardiac vertical muscles in Scorpio, and eight in Limulus (see
gs. 30, 31, 32). It is obvious that the contraction of these muscles must cause a depression of the floor of the pericardium and a rising of the roof of the ventral blood sinus, and a consequent increase of volume and flow of blood to each. Whether the pericardium and the ventral sinus are made to ex and simultaneously or all the movement is made b one only of the surfaces concerned, must depend on conditions 0 tension. in any case it is clear that we have in these muscles an apparatus for causing the blood to flow differentially in increased volume into either the pericardium, through the veins leading from the respiratory organs, or from the body generally into the great sinuses which bring the blood to the respiratory organs. These muscles act so as to pump the blood through the respiratory 0 ans.
t is not surprising that with so highly developed an arterial system Limulus and Scorpio should have a highly developed mechanism for determining the flow of blood to the respiratory organs. That this is, so to speak, a need of animals with localized respiratory
organs is seen by the existence of provisions servin a similar purpose in other animals, e.g. the branchial hearts of the ephalopoda.
The veno-pericardiac muscles of Scorpio were seen and fi ured by Newgort but not described b him. Those of Limulus were escribed and gured by Alphonse Mi ne-Edwards, but he called them merely “ transparent ligaments," and did not discover their muscular structure. The are figured and their importance for the first time recognized in t e memoir on the muscular and skeletal systems of Limulus and Scor io by Lankester, Beck and Bourne (4).
6. Alimentary anal and Gastric Glands.—,The alimentary canal in Scorpio, as in Limulus, is provided with a powerful suctorial pharynx, in the working of which extrinsic muscles take a part. The mouth is relatively smaller in Scorpio than in Limulus—in fact is minute, as it is in all the terrestrial Arachnida which suck the juices of either animals or plants. In both, the alimentary canal takes a strai ht course from the pharynx (which bends under it downwards and ackwards towards the mouth in Limulus) to the anus, and is a simple. narrow, cylindrical tube (fig. 33). The only point in which the gut of Limulus resembles that of Scorpio rather than that of any of the Crustacea, is in possessing more than a single pair of ducts or lateral outgrowths connected with ramified gastric glands or gastric caeca. Limulus has two pairs of these, Scorpio as man as six pairs. The Crustacea never have more than one pair. e minute microscopic structure of the gastric glands in the two animals is practicall identical. The functions of these gastric diverticula have never n carefully investigated. It is very probable that in Scorpio they do not serve merely to secrete a di estivc fluid (shown to other Arthropoda to resemble the pancreatic uid), but that they
also become distended by the juices of the rey sucked in by the scorpion—as certainly must occur in the case 0 the simple unbranched gastric caeca of the spiders.
The most im rtant difference which exists between the structure of Limulus an that of Scorpio is found in the hinder region of the alimentary canal. Scorpio is here provided with a single or double pair of renal excretor tubes, which have been identified by earlier authors with the Mallpighian tubes of the Hexa d and Myriapod insects. Limulus is devoid of any such tubes. e shall revert to this subject below.
7. Ovaries and Spermaries: Conacocls and Ganoducts.—The scorpion is remarkable for having the specialized portion of, coelom from the walls of which egg-cells or sperm-cells are developed according to sex, in the form of a simple but extensive network. It is not a ir of simple tubes, nor of dendriform tubes, but a closed network. be, same fact is true of Limulus. as was shown by Owen (7)
FIG. 27.——Diagram showing the position of the coxal glands of a scorpion, Buthus australis, Lin., in relation to the legs, diaphragm (cntosternal flap), and the gastric caeca.
l to 6, The bases of the six pro-
(sometimes called salivary). B, Coxal gland.
C, Diaphragm of Newport =fibrous flap of the entosternum. D, Mesosomatic gastric caeea
(so~called liver). .
E, Alimentary canal.
(From Lankester, Q. 1. Mia. Sci, vol. xxiv. N.S. p. 152.)
in regard to the ovary, and by Benham (14) in regard to the testis. This is a very definite and remarkable agreement, since such a reticular onocoel is not found in Crustacea (except in the male Apus). hioreover, there is a significant agreement in the character of the spermatozoa of Limulus and Scorpio. The Crustacea are—with the exception of the Cirrhi edia—remarkable for having stifl', motionless spermatozoids. In fimulus Lankester found (15) the spermatozoa to possess active fla elliform “ tails," and to resemble very closely those of Scorpio whic , as are those of most terrestrial Arthropoda, are a-~tively motile. This is a microscopic point of agreement. but is none the less significant.
In regard to the important structures concerned with the fertilization of the egg, Limulus and Scorpio differ entirely from one another. The eggs of Limulus are fertilized in the sea after they have been laid. Scorpio, being a terrestrial animal, fertilizes by copulation. The male possesses elaborate copulatory structures of a chitinous nature, and the e gs are fertilized in the female without even quitting the place where they are formed on the wall of the reticular gonocoel. The female scorpion is viviparous, and the young are produced in a highly developed condition as fully formed scorpions.
Diflerences between Limulus and Scorpio—We have now passed in review the principal structural features in which Limulus agrees with Scorpio and differs from other Arthropoda. There remains for consideration the one important structural difference between the two animals. Limulus agrees with the majorit of the Crustacea in being destitute of renal excretory caeca or tubes opening into the hinder part of the gut. Sconpio, on the other hand, in common
Fro. 28.—The right coxal ' gland of Limulus polyphemus,
a’ to a“, Posterior borders of the chitinous bases of the coxae of the second, third, fourth and fifth prosomatic limbs.
b, Longitudinal lobe or stolon of the coxal gland.
c. lts four transverse lobes or outgrowths corresponding to the four coxae.
with all air-breathin Arthropoda except Peripatus. possesses these tubules, which are 0 ten called Malpighian tubes. A great deal has been made of this difierence by some writers. It has been considered by them as proving that Limulus, in spite of all its special agreements with Scorpio (which, however, have scarcel been appreciated by the writers in question), really belon s to the rustacean line of descent, whilst Scorpio, by possessing alpighian tubes, is declared to be unmistakably tied together With the other Arachnida to the tracheate Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess Malpighian tubes.
It must be pointed out that the presence or absence of such renal excretory tubes opening into the intestine appears to be a question
FIG. 29.—Dia ram of the arterial system 0 A, Scorpio, and B, Limulus. The Roman numerals indicate the body somites and the two figures are adjusted for comparison. ce, Cerebral arteries; 3;», supra-spinal or medullary artery; c, caudal artery; 1, lateral anastomotic arter of Limulus. The figure also shows the peculiar neural investiture formed b the cerebral arteries in Limulus and the derivation from this of the arteries to the limbs, III, IV, VI, whereas cin Scorpio the latter have a separate origin from the anterior aorta.
(From Lankester, Aracnnid.)
of adaptation to the changed physiological conditions of respiration, and not of morphological si nificance, since a pair of renal excretory tubes of this nature is foun in certain Amphipod Crustacea (Talorchestia, &c.) which have abandoned a purely a uatic life. This view has been accepted and supported by Professors orschelt and Heider (16). An important fact in its favour was discovered by Laurie (17), who investigated the embryology of two species of Scorpio under Lankester's direction. It a pears that the Malpighian tubes of Scorpio are developed from t e mescnteron, viz. that portion of the gut which is formed by the hypoblast, whereas in Hexapod insects the similar caecal tubes are developed from the proctodaeum or impushed portion of the gut which is formed from epiblast. In fact it is not possible to maintain that the renal excretory tubes of the gut are of one common origin in the Arthropoda. They have appeared independently in connexion with a change in the excretion o nitrogenous waste in Arachnirls, Crustacea, and the other classes of Arthropoda when aerial, as opposed to aquatic, respiration has been esta lishedfand they have been formed in some cases from the mesenteron, in' other cases from the proctodaeum. Their appearance In the air-breathing Arachnids does not separate those forms from the water-breathing Arachnids which are devoid of them,
any more than does their appearance in certain Amphipoda separate those Crustaceans from the other members of the class.
Further, it is pointed out by Korschelt and Heider that the hinder portion of the gut frequentl acts in Arthropoda as an organ of nitrogenous excretion in the a sence of any special excretory tubules, and that the production of such caeca from its surface in separate lines of descent does not inVOlve any elaborate or unlikely process of
rowth. In other words, the Malpighian tubes of the terrestrial
rachnida are homoplastic with those of Hexapoda and Myriapoda, and not homogenetic with them. We are compelled to take a similar view of the agreement between the tracheal air-tubes of Arachnida and other tracheate Arthropods. They are homo lasts (see 18) one of another, and do not owe their existence in t e various classes compared to a common inheritance of an ancestral tracheal system.
Conclusions arising from the Close A finin of Limulus and Scorpio—When we consider the relationships of the various classes of Arthropoda, having accepted and established the
(From Lankestcr, lac. cil , after
fact of the close genetic affinity of Limulus and Scorpio, we are led to important conclusions. In such a consideration we have to make use not only of the fact just mentioned, but of three important generalizations which serve as it were as implements for the proper estimation of the relationships of any series of organic forms. First of all there is the generalization that the relationships of the various forms of animals (or of plants) to one another is that of the ultimate twigs of a much-branching genealogical tree. Secondly, identity of structure in two organisms does not necessarily indicate that the identical structure has been inherited from an ancestor common to the two organisms compared (homogeny), but may be due to independent development of a like structure in two different lines of descent (homoplasy). Thirdly, those members of a group which, whilst exhibiting undoubted structural characters indicative of their proper assignment to that group, yet are simpler than and inferior in elaboration of their organization to other members of the group, are not necessarily representatives of the earlier and primitive phases in the development of the group—but are very often examples of retrogressive change or degeneration. The second and third implements of analysis above cited are of the nature of
Fro. 30.—View from below of
a scorpion (Buthus occitanus)
opened and dissected so as to
s ow the pericardium with its
muscles, the lateral arteries, and
the te o-sternal muscles.
dpm, Dorso-plastral muscle.
art, Lateral artery.
tsm‘, Tergo-sternal muscle (labelled do in fig. 3!) of the second (pectiniferous) mesosomatic somite; this is the most anterior pair of the series of six, none are present in the genital somite.
tsm‘, Tergo-sternal muscle of the fifth mesosomatic somite.
ism“, Tergo-stemal muscle of the enlarged first metasomatic somite.
Vl-"Ml to VPM’, The series of seven pairs of vcno'pcricardiac muscles (labelled pr in fig. 31). There is some reason to admit
the existence of another more
anterior pair of these muscles in
Scor io; this would make the
num er exactly correspond with
the number in Limulus.
cautionsorchecks. Agreements . vol. XL, itB3.)
are not necessarily due to common inheritance; simplicity is not necessarily primitive and ancestral.
On the other hand, we must not rashly set down agreements as due to “ homoplasy ” or “ convergence of development ” if we find two or three or more concurrent agreements. The probability is against agreement being due to homoplasy when the agreement involves a number of really separate (not correlated) coincidences. Whilst the chances are in favour of some one homoplastic coincidence or structural agreement occurring between some member or other of a large group a and some member or other of a large group b, the matter is very different when by such an initial coincidence the two members have been particularized. The chances against these two selected members exhibiting another really independent homoplastic agreement are enormous: let us say 10,000 to r. The chances against yet another coincidence are a hundred million to one, and against yet one more “ coincidence ” they are the square of a hundred million to one. Homoplasy can only be assumed when the coincidence is of a simple nature, and is such as may be reasonably supposed to have arisen by the action of like selective conditions upon like material in two separate lines of descent.l
So, too, degeneration is not to be lightly assumed as the explanation of a simplicity of structure. There is a very definite criterion of the simplicity due to degeneration, which can in most cases be applied. Degenerative simplicity is never uniformly distributed over all the structures of the organism. It affects many or nearly all the structures of the body, but leaves some, it may be only one, at a high level of elaboration and complexity. Ancestral simplicity is more uniform, and does not co-exist with specialization and elaboration of a single organ. Further: degeneration cannot be inferred safely by the examination of an isolated case; usually we obtain a series of forms indicating the steps of a change in structurehand what we have to decide is whether the movement has been from the simple to the more complex, or from the more complex to the simple. The feathers of a peacock afford a convenient example of primitive and degenerative simplicity. The highest point of elaboration in colour, pattern and form is shown by the great eye-painted tail feathers. From these we can pass by gradual transitions in two directions, viz. either to the simple lateral tail feathers with a few rami only, developed only on one side of the shaft and of uniform metallic coloration—or to the simple contour feathers of small size, with the usual symmetrical series of numerous rami right and left of the shaft and no remarkable colouring. The one-sided specialization and the peculiar metallic colouring of the lateral tail feathers mark them as the extreme terms of a degenerative series, whilst the symmetry, likeness of constituent parts inter se, and absence of specialized pigment, as well as the fact that they differ little from any average feather of birds in general, mark the contour feather as primitiver simple, and as the starting-point from which the highly elaborated eye-painted tail feather has gradually evolved.
Applying these principles to the consideration of the Arachnida, we arrive at the conclusion that the smaller and simpler Arachnids are not the more primitive, but that the Acari or mites are, in fact, a degenerate group. This was . maintained by Lankester in 1878 (19), again in 1881 (20); it was subsequently announced as a novelty by Claus in 1885 (21). Though the aquatic members of a class of animals are in some instances derived from terrestrial forms, the usual transition is from an aquatic ancestry to more recent land-living forms. There is no doubt, from a consideration of the facts of structure, that the aquatic water-breathing Arachnids, represented in the past by the Eurypterines and to-day by the sole survivor Limulus, have preceded the terrestrial air-breathing forms of that group. Hence we see at once that the better-known Arachnida form a series, leading from Limulus-like aquatic creatures through scorpions, spiders and harvest-men, to the degenerate Acari or mites. The spiders are specialized and reduced in apparent complexity, as compared with the scorpions, but they cannot be regarded as degenerate since the concentration of structure which occurs in them results in greater efficiency and power than are exhibited by the scorpion. The determination of the relative degree of perfection of organization attained by two animals
‘ A great deal of superfluous hy thesis has lately been put forward in the name of “ the principle oimconvergence of characters " by a certain school of palaeontologists. The horse is supposed by these writers to have originated by separate lines of descent in the Old World and the New, from five-toed ancestors! And the important consequences following from the demonstration of the identity in structure of Limulus and Scorpio are evaded by arbitrary and even phantastic invocations of a mysterious transcendental force
which bringal about “convergence” irrespective of heredity and selection. orfhology becoms a farce when such assumptions are made. (E. R. .)
After Benlmm, Trans. Zool. Soc. vol. xi., 1883.
Leaving that question for consideration in connexion with the systematic statement of the characters of the various groups of Arachnida which follows on p. 299, it is well now to consider the following question, viz., seeing that Limulus and Scorpio are such highly developed and specialized forms, and that they seem to constitute as it were the first and second steps in the series of recognized Arachnida—what do we know, or what are we led to suppose with regard to the more primitive Arachnida from which the Eurypterines and Limulus and Scorpio have sprung P Do we know in the recent or fossil condition any such primitive Arachnids? Such a question is not only legitimate, but prompted by the analogy of at least one other great class of Arthropods. The great Arthropod class, the Crustacea, presents to the zoologist at the present day an immense range of forms,
comprising the primitive phyllopods, the minute copepods, the parasitic cirrhipedes and the powerful crabs and lobsters, and the highly elaborated sand-hoppers and slaters. It has been insisted, by those who accepted Lankester’s original doctrine of the direct or genetic affinity of the Chaetopoda and Arthropoda, that Apus and Branchipus really come very near to the ancestral forms which connected those two great branches of Appendiculate (Parapodiate) animals. On the other hand, the land crabs are at an immense distance from these simple forms. The record of the Crustacean family
tree is, in fact, a fairly complete
7"" one—the lower primitive members 'n of the group are still represented ‘__’-. g' by living forms in great abundance. r, In the case of the Arachnida, if we I‘ 2 have to start their genealogical _"c history with Limulus and Scorpio, ‘\‘ we are much in the same position 4" as we should be in dealing with the Crustacea, were the whole of the Entomostraca and the whole of the Arthrostraca wiped out of existence and record. There is no possibility of doubt that the series of forms corresponding in the Arachnidan line of descent, to the forms disPm tinguished in the Crustacean line B of descent as the lower grader—the Entomostraca—have ceased to exist, and not only so, but have left little evidence in the form of fossils as to their former existence and nature. It must, however, be admitted as probable that we should find some evidence, in ancient rocks or in the deep sea, of the early more primitive Arachnids. And it must be remembered that such forms must be expected to exhibit, when found, differences from Limulus and Scorpio as great as those which separate Apus and Cancer. The existing Arachnida, like the higher Crustacea, are “ nomomeristic,” that is to say, have a fixed typical number of somites to the body. Further, they are like the higher Crustacea, “ somatotagmic,” that is to say, they have this limited set of somites grouped in three (or more) “ tagmata " or regions of a fixed number of similarly modified somites
From Lankester, "Limulus an Arachnid."
Flo. 33.—The alimentary canal and gastric glands of a scorpion (A) and of Limulus (B). in, Muscular suctorial en
la ement of the pharynx. sal, rosomatic pair of gas
tric caeca in Scorpio, called salivary glands by some writers.
c‘, and c’, The anterior two pairs of gastric caeca and ducts of the mesosomatic re ion.
0', c‘ and c“, aeca and ducts of Scorpio not represented in Limulus.
M, The Malpighian or renal
portion of gut leading to anus and formed embryologically by an inversion of the epiblast at that orifice.
from that characterizing a neighbouring “ tagma.” The most primitive among the lower Crustacea, on the other hand, for example, the Phyllopoda, have not a fixed number of somites, some genera—even allied species—have more, some less, within wide limits; they are “anomomeristic.” They also, as is generally the case with anomomeristic animals, do not exhibit any conformity to a fixed plan of “ tagmatism ” or division of the somites of the body into regions sharply marked off from one another; the head or prosomatic tagma is followed by a trunk consisting of somites which either graduate in character as we pass along the series or exhibit a large variety in different genera, families and orders, of grouping of the somites. They are anomotagmic, as well as anomomeristic.
When it is admitted—as seems to be reasonable—that the primitive Arachnida would, like the primitive Crustacea, be
anomomeristic and anomotagmic, we shall not demand of claimants for the rank of primitive Arachnids agreement with Limulus and Scorpio in respect of the exact number of their somites and the exact grouping of those somites; and when we see how diverse are the modifications of the branches of the appendages both in Arachnida and ih other classes of Arthropoda. (q.v.), we shall not over-estimate a difierence in the form of this or that appendage exhibited by the claimant as compared with the higher Arachnids. With those considerations in mind, the claim of the extinct group of the trilobites to be considered as representatives of the lower and more primitive steps in the Arachnidan genealogy must, it seems, receive a favourable judgment. They differ from the Crustacea in that they have only a single pair of prae-oral appendages, the second pair being definitely developed as mandibles. This fact renders their association with the Crustacea impossible, if classification is to be the expression of genetic afiinity inferred from structural coincidence. On the contrary, this particular point is one in which they agree with the higher Arachnida. But little is known of the structure of these extinct animals; we are therefore compelled to deal with such special points of resemblance and difference as their remains still exhibit. They had lateral eyes‘ which resemble no known eyes so closely as the lateral eyes of Limulus. The general form and structure of their prosomatic carapace are in many striking features identical with that of Limulus. The trilobation of the head and body—due to the expansion and flattening of the sides or “pleura” of the tegumentary skeleton~is so closely repeated in the young of Limulus that the latter has been called “ the trilobite stage ” of Limulus (fig. 42 compared with fig. 41), No Crustacean exhibits this trilobite form. But most important of the evidences prmented by the trilobites of affinity with Limulus, and therefore with the Arachnida, is the tendency less marked in some, strongly carried out in others, to form a pygidial or telsonic shield—a fusion of the posterior somites ot the body, which is precisely identical in character with the metasomatic carapace of Limulus. When to this is added the fact that a post-anal spine is developed to a large size in some trilobites (fig. 38), like that of Limulus and Scorpio, and that lateral spines on the pleura of the somites are frequent as in Limulus, and that neither metasomatic fusion of somites nor post-anal spine, nor lateral pleural spines are found in any Crustacean, nor all three together in any Arthropod besides the trilobites and Limuluse—the claim of the trilobites to be considered as representing one order of a lower grade of Arachnida, comparable to the grade Entomostraca of the Crustacea, seems to be established.
The fact that the single pair of prae-oral appendages of trilobites, known only as yet in one genus, is in that particular case a pair of uni-ramose antennae—does not render the association of trilobites and Arachnids improbable. Although the prae-oral pair of appendages in the higher Arachnida is usually chelate, it is not always so; in spiders it is not so; nor in many Acari. The bi-ramose structure of the post-oral limbs, demonstrated by Beecher in the trilobite Triarthrus, is no more inconsistent with its claim to be a primitive Arachnid than is the foliaceous modification of the limbs in Phyllopods inconsistent with their relationship to the Arthrostracous Crustaceans such as Gammarus and Oniscus.
Thus, then, it seems that we have in the trilobites the representatives of the lower phases of the Arachnidan pedigree. The simple anomomeristic trilobite, with its equi-forrnal somites and equi-formal appendages, is one term of the series which ends in the even more simple but degenerate Acari. Between the two and at the highest point of the arc, so far as morphological differentiation is concerned, stands the scorpion; near to it in the trilobite’s direction (that is, on the ascending side) are Limulus and the Eurypterines—with a long gap, due to obliteration of the record, separating them from the trilobite. On the
‘ A pair of round tubercles on the labrum (camerostome or h £0stoma) of several species of Trilobites has been described and lief, to be a pair of eyes (22). Sense-organs in a similar position were discovered in Limulus by Patten (42) in 1894.