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 ARACHNIDA

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several separate small eyes. The microscopic structure Following the prosoma is a region consisting of six of the central and the lateral eyes has been shown by segments (Figs. 14 and 15), each carrying a pair of plateLankester and Bourne (5) to differ; but the lateral eyes like appendages in both Limulus and Scorpio. This region is called the mesosoma. The tergites of this fo region and those of the following region, the metasoma, are fused to form a second or posterior carapace in Limulus, whilst remaining free in Scorpio. The first pair of foliaceous appendages in each animal is the genital operculum; beneath it are found the openings of the genital 1C a ducts. The second pair of mesoFig. 5.—Entostemum of one of the Fig. 6.—Ventral surface of the same somatic appendages in Scorpio aie C3 mygalomorphous spiders; dorsal entostemum as that drawn in. known as the “ pectens.” Each con surface. Ph.N., pharyngeal notch. Fig. 5. Ph.N., pharyngeal notch. The three pairs of rod-like tendons Behind it on the body of the sists of an axis, bearing numerous correspond to the two similar pairs entosternum is seen the neural blunt tooth-like processes arranged in Limulus, and the posterior fossa, as in Fig. 2. (After Lan- in a series. This is represented in rxiv median process with its repetition kester, loc. cit.) of triangular segments closely Limulus by the first gill bearing resembles the same process in Limulus. Magnified five times appendage. The leaves (some 150 xv linear. (From Lankester, loc. dt.) in number) of the gill - book (see of Scorpio were shown by them to be similar in structure figure) correspond to the tooth-like to the lateral eyes of Limulus and the central eyes of processes of the pectens of Scorpio. XVI Scorpio to be identical in structure with the central eyes The next four pairs of appendages of Limulus (see below). (completing the mesosomatic series of six) consist, in both Scorpio if xvn and Limulus, of a base carrying each 130 to 150 blood-holding, leaf-like plates, lying on one anMI other like the leaves of a book. Their minute structure is closely similar in the two cases; the leafPA like plates receive blood from the great sternal sinus, and serve as respiratory organs. The difference Fig. 8.—Diagram of the between the gill-books of Limulus dorsal surface of a Scorto compare with and the lung-books of Scorpio de- pion Fig. 7. Letters and Roman numerals as in pends on the fact that the latter 7, excepting that VII are adapted to aerial respiration, Fig. is here certainly the tergum of the first somite while the former serve for aquatic of the mesosoma — the respiration. The appendage carry- genital somite —and is not survival of the embrying the gill-book stands out on aonic prsegenital somite. the surface of the body in Limulus, (From Lankester, loc. cit.) The anus (not seen) is on and has other portions developed the sternal surface.

XH 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 m appendage-bearing somites of the prosoma; VII, usually considered to be the tergum of the genital somite, but suggested by Mr. Pocock to be that of the HVtoXVll! otherwise suppressed prsegenital somite; VIII to XIII, the six somites of the Anus mesosoma, each with a movable pleural spine and a pair of dorsal entopophysis or muscle-attaching ingrowths ; XIV to XVIII, the confluent or unexpressed six somites of the metasoma. (From Lankester, Q. J. Micr. Sci. vol. xxi. 1881.) Fig. 9.—Ventral view of the posterior carapace or meso-meta-somatic According to the system of numbering explained in the text, if VII is the tergum (opisthosomatic) fusion of Limulus polyphemus. The soft integument of the prsegenital somite (as is probable) it should be labelled Prg without any and limbs of the mesosoma have been removed as well as all the viscera number, and the somites VIII to XIII should be lettered 1 to 6, indicating that and muscles, so that the inner surface of the terga of these somites with they are the six normal somites of the mesosoma ; whilst XV to XVIII should be their entopophyses are seen. The unsegmented dense chitinous sternal replaced by the numbers 7 to 12—an additional suppressed segment (making up plate of the metasoma (XIII to XVTII) is not removed. Letters as in Fig. 7. (After Lankester, loc. cit.) the typical six) being reckoned to the metasomatic fusion. S. I. — 66