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 ARACHNIDA sixth leg-bearing somites of the prosoma and indicate the appendages by the Roman numerals, I., II., III., IV., V., VI., and whilst ignoring the prsegenital 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 from other Arthropoda. rdiffers cam i1]ie chief of these are as -—M follows :— 1. The Composition of the •pmst Head (that is to say, of the anterior part of the prosoma) with especial Reference to the mets Region in Front of the Mouth.— It appears (see Arthropoda) Fia. 20.-View of the ventral that there is embryological evisurface of the mid-line of the f the existence of two prosomatic region of Limulus . 0., . .. . poiyphemus. The coxae of the somites m Arachnida which were originally post-oral, but have a series on each side between become rprse-oral by adaptational the mouth, M, and the meta-. ... o i sternites, mets. sf, The sub- shifting of the oral aperture. frontal median sclerite ; Ch, rppggg forwardly-slipped SOmiteS 11 the chelicerse; com, the camero. ,, stome or upper lip; M, the are called prosthomeres. I he mouth; pmst, the promeso- llrst /•, 01<> flneSe i u • Arachnids sternal sclerite or chitinous in Aratlimuh plate, unpaired ; mets, the right ag in other Arthropods, its pair and left metasternites (corre- o -i ,„i spending to the similarly placed of appendages represented by the pentagonal sternite of Scorpio. eyes> The second has for its pair Naturalsize. (AlterLankester.) J n • r of appendages the small pair ot limbs which in all living Arachnids is either chelate or retrovert (as in spiders), and is known as the chelicerse. 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 chelicerse of Limulus and of the ganglionic nerve-masses from which they receive their nervesupply, is closely similar to that of the same strucch,

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done as much for Scorpio. Limulus thus agrees with Scorpio and differs from the Crustacea, in which there are three prosthomeres—one ocular and two carrying palpiform appendages. It is true that in the lower Crustacea (Apus, Ac.) we have evidence of the gradual movement forward of the nerve-ganglia belonging to these 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 posilens tion of two appendage-bearing somites in addition to the ocular prosthomere. The Crustacea have, in fact, three prosthomeres in the head and the Arachnida only two, and Limulus agrees with the Arachnida in this respect and differs Fig. 22. — Section through the lateral eye of from the Crus- retinal Euscorpius nerv.c,; cellsitalicus. (nerve-endZ_e?is,_Cuticular cells) ; rhabd, lens; rhabdomes nerve fibres of the optic nerve ; int, intertacea. The central nerv.f, mediate cells (lying between the bases of the nervous systems retinal cells). (After Lankester and Bourne from Parker and Haswell’s Text-book of Zoology, Macof Limulus and millan and Co.) of Scorpio present closer agreement in structure than can be found when a Crustacean 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 properly belong to hinder segments, are very nearly identical in the two animals. The form and disposition of the ganglion cells are also peculiar and closely similar in the two. (See Patten (42) for important observations on the neuromeres, Ac., of Limulus and Scorpio.)

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.) tures in Scorpio. The cerebral mass is in Limulus more easily separated by dissection as a median Fl0 23 Section through a portion of the lateral eye of Limulus, showing three ommalobe distinct from the laterally-placed ganglia of the tidia—A, B, and C. hyp, The epidermic cell-layer (so-called hypodermis), the cells of increase in volume below each lens, l, and become nerve-end cells or retinulacheliceral somite than is the case in Scorpio, but the which cells, rf ; 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, relations are practically the same in the two forms. chitinous cuticle. (From Korschell and Heider after Watase.) Formerly it was supposed that in Limulus both the chelicerse and the next following pair of appendages were 2. The Minute Structure of the Central Eyes and of the prosthomerous, as in Crustacea, but the dissections of Lateral Eyes.—Limulus agrees with Scorpio not only in Alphonse Milne-Edwards (6) demonstrated the true limita- having a pair of central eyes and also lateral eyes, but in tions of the cerebrum, whilst embryological researches have the microscopic structure of those organs which differs in