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

the last pair of legs in both Scorpio and Limulus, viz. the pentagonal 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."

M

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:

met

y

III

-VI

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.

Mouth. It

to the Region in Front of the M, Mouth behind the oval median appears (see camerostome. ARTHROPODA) that there is I, The chelicerae. embryological evidence of II, The chelae.

somites.

soma.

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 somite of the mesosoma with the have become prae-oral by genital operculum (a fused pair of adaptational shifting of the limbs). oral aperture. forwardly-slipped somites IXstg to XIIstg, the four pulmonary These VIII, The pectiniferous somite. are called "prosthomeres.' The first of these has, in met, The pentagonal metasternite of Arachnids as in other the prosoma behind all the coxae. Arthropods, its pair of ap- x, The sternum of the pectiniferous pendages represented by somite. the eyes. The second has for its pair of appendages y, The broad first somite of the metathe 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. 1.-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 following pair of appendages were prosthomerous, as in Crustacea, it was supposed that in Limulus both the chelicerae and the next but the dissections of Alphonse Milne-Edwards (6) demonstrated

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.

gp, Genital pore.

(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.

ม

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 I 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

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 tendinous outgrowths standing out from it at right angles on each side. It "floats 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 entoster

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

appendages.

sgc, Frontal groove.

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 abp2, Rudiment of the right half of the entosternite the approxithe genital operculum. mation of the nerve cords took abp3, Rudiment of the right pecten. place in the prosoma, and thus abp to abp, Rudiments of the four they were able to take up a appendages which carry the pul- position below the entosternite. monary lamellae. In the mesosoma the approximation had occurred before the entosternites were formed.

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

In the scorpion (figs. 3 and 4) the entosternite has tough VIII, The first mesosomatic somite membrane-like outgrowths or genital somite. which connect it with the

In Mygale (figs. 5 and 6) the form of the entosternite is more like that of Limulus than is that of Scorpio.o The anterior notch Ph.N. is similar orqa to that in Limulus, whilst the imbri-mod fwenslefui 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. Tova moda ve Isisomite, 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 (After Brauer, loc. cit.) 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 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 l fibres connected with the suctorial old

pharynx are in Limulus insertedoing

into the entosternite, and the abys activity of the two organs may be

1895.)

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

heart differs in Limulus from the arrangement obtaining in Scorpio, in that a pair of lateral commissural arteries exist in Limulus (as described by Alphonse Milne-Edwards (6)) leading to a suppression of the more primitive direct connexion of the four pairs of posterior Y. 1.

V.

mes.

FIG. 21. Development of the lateral eyes of a scorpion. h, Epidermic cell-layer; mes, mesoblastic connective tissue; n, nerves; II, III, IV, V, depressions of the epidermis in each of which a cuticular lens will be formed.

(From Korschelt and Heider, after Laurie.)

lateral arteries and of the great median posterior arteries with the heart itself (fig. 29). The arterial system is very completely developed in both Limulus and Scorpio, branching repeatedly until minute arterioles are formed, not to be distinguished from true capillaries;

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 1883 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 peculiar to that group, would have considerable weight as a proof of the close genetic affinity of Limulus and Scorpio.

FIG. 23.-Section through a portion of the lateral eye of Limulus, showing three ommatidia-A, B and C. hyp, The epidermic cell-layer (so-called hypodermis), the cells of which increase in volume below each lens, I, and become nerve-end cells or retinula-cells, rt; in A, the letters rh point to a rhabdomere secreted by the cell rt; c, the peculiar central spherical cell; n, nerve fibres; mes, mesoblastic skeletal tissue; ch, chitinous cuticle.

The great pericardial sinus is strongly developed in both animals. Its walls are fibrous 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 large veins, which bring the blood respectively from the gill-books and the 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 great 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 of these venopericardiac vertical muscles in Scorpio, and eight in Limulus (see figs. 30, 31, 32). It is obvious that the contraction of these muscles