ARACHNIDA 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 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, Palsemon) 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
Fig. 26.—A, diagram of a retinula of the central eye of a Scorpion consisting of five retina-cells (ref.), with adherent branched pigment cells (pig.)', B, rhabdom of the same, consisting of tive confluent rhabdomeres ; C, transverse section of the rhabdom of a retinula of the Scorpion’s central eye, showing its five constituent rhabdomeres as rays of a star ; D, transverse section of a retinula of the lateral eye of Limulus, showing ten retinula cells, ret., each bearing a rhabdomere, rhab. (After Lankester.) 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
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same in essentials as that of the coxal glands of Scorpio (13). Coxal glands have since been recognized and described in other Arachnida. It has lately (1900) been 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 (13b) 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 greenglands 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. Fig. 27.—Diagram showing the posiBoth belong to the category tion of the coxal glands of a ScorButhus australis, Lin., in relaof “ coelomoducts,” namely, pion, tion to the legs, diaphragm (entotubular or funnel-like por- sternal flap), and the gastric cseca. to 6, The bases of the six prosotions of the coelom opening 1matic limbs ; A, prosomatic gastric (sometimes called salivary); to the exterior in pairs in gland B, coxal gland ; C, diaphragm of each somite (potentially), and Newport=fibrous flap of the entosternum ; D, mesosomatic gastric usually persisting in only a caeca (so - called liver); E, alimentary canal. (From Lankester, few somites as either “ uroQ. J. Mic. Sci. vol. xxiv. N.S. p. coels ” (renal organs) or 152.) “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 Chaffopod 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 they 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 Entosternites and their Minute Structure.—StrausDurkheim (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. 1 to 6) the entosternites of Limulus, Scorpio, and Mygale. Lankester some years ago made a special study of the histology
