Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/683

Rh CRUSTACEA 649 Dohrn l and Dr A. S. Packard. 2 The natural history of the king-crab has been studied by the Rev. Samuel Lockwood, 3 and its anatomy has quite recently formed the subject of two elaborate memoirs by Professor Owen, 4 and by Dr Alphonse Milne-Edwards. 5 Ve can only very briefly notice these important contribu tions to our knowledge of the Xiphosura here. The female Limulus of the north-east American coast spawns twice every year during the months of May, June, or July, 6 at the great high tides. It comes lip to near high-water mark, spawning under water ; thus the eggs are FIG. 4fi. 7 Eggof Limulus polyphemus: a, protoderm; 5, thechorion (after Dohrn). FIG. 47. Third stage in the embryo of Limulus:, protoderm ; 2&amp;gt;,chorion (after Packard). FIG. 48. Fourth stage (?) in the embryo of Limulus (after Dr. Packard s figure). FIG. 49. Fourth stage (?) in the embryo of Limulus: 1, antennule; 2, antenna; 3-6, maxillipedes ; 7 and 8, thoracic plates afterwards bearing the branchiae ; m, the mouth; x, the ovarian apertures (?) ; a, the abdomen (after Dohrn). daily exposed to the sun s warmth for a short time at low water. Great numbers arrive in pairs, the male grasping the sides of the shield of the female with his strong and peculiarly modified chelate antennae. The eggs are deposited by the female in a hole in the sand, and are fecundated by the male after deposition, and are then left to hatch. Only one other similar case is on record, namely, that of the common freshwater cray-fish, in which, according to M. Chantran, the eggs are fecundated after expulsion from the oviducts. The eggs occupy from fifty to seventy days in hatching, according to the favourable or unfavour able conditions under which they are deposited ; some which Dr Lockwood set aside in a jar of sea- water in a dark place hatched after 350 days ! The egg has two membranes, a dense inelastic chorion and an inner elastic protoderm. 8 This chorion remains entire so long as development is arrested or is sluggish, but 1 &quot;Zur Embryologie nnd Morphologic des Limulus polyplwmus&quot; in Jenaischen Zcitschrift, Band vi. Heft 4, Taf. xiv. and xv., 1871. 2 &quot; The Development of Limulus polyphemus,&quot; by A. S. Packard, in Memoirs oj Boston Soc Nat. Hist. 1871, vol. ii. pp. 155-202, pi. 3-5. 3 &quot;The Horse-Foot Crab,&quot; by the Rev. S. Lockwood, in American Naturalist, 1870, vol. iv. p. 257. 4 &quot; Anatomy of the American King-Crab,&quot; by Prof. Owen, in Trans. Linn. Soc. 1872, vol. xxviii. pp. 459-506, pi. 36-39. 5 Etudes sur les Xiphosures et Us Crustacea de la Region Mexicaine, par Alph. Milne- Edwards (Paris, 1873, folio, pp. 43. pi. 1-12). 6 These investigations are confined to the American king-crab, and were made at Raritan Bay, New Jersey. Van der Hoeven s memoir on Limulus was written on the East Indian Limulus moluccanus (Reck, sur I Hist. Nat. et Anatom. des Limules, fol. 1838, Leyden, p. 48, plates 1-7). 7 Figs. 46-55 are from H. Woodward s paper on the &quot; Relationship of the Xiphosura to the Eurypterida, &e.,&quot; Quart. Journ. Geol. Soc. 1872, vol. xxviii. p. 50. 8 Dohrn calls the inner membrane in the egg of Limulus the &quot; chorion&quot; and the outer the exochorion, but Packard s tei-m &quot;protoderm &quot; appears pre erable for the former. as soon as the embryo increases in size, the tough chorion splits asunder, and the inner elastic protoderm enlarges, becomes dense, and vicariously fulfils the duties of the former (fig. 46). A similar splitting of the external egg-membrane has been noticed in Apus. Fritz Miiller points out that in some Isopoda (as for instance Philoscia) the larval skin is not only without any folds or sac-like diverticula, but is closely applied to the egg-membrane. This second egg- membrane in Limulus may perhaps therefore correspond to this first larval skin. Certainly, when the embryo first appears, its position is the same as in Aselhis, Ligia, ridloscia, and other Isopcds, i.e., with its ventral sur- 51 FIG 50 Fifth stage (?) of embryo of Limulus (after Dohrn). At this stage the chorion is split, and the protoderm is expanded by the admi&amp;gt;s,on of water by endosmose, in which the embryo is seen to revolve. p^vnrm. FIG. 51,-NInth stage (?) of embryo, &quot;just before hatching (after Packaid); dorsal aspect FIG. 52. The same : side view of embryo. FIG. 53.-Larva of Limulus recently hatched (after Packard). FIG 54.-Larva of Limulus on hatching (the &quot; Tnlobnenstadium of Doh.n). FIG. to ZWawsfew ornatus, Siernb. ; adult specimen with six thoracic segments and fully-developed genal spines. face convex (figs. 47 and 48). In its first stage _the larval Limulus has six bud-like indications on each side of the mesial line, where the paired cephalic appendages will be developed (fig. 47). In later stages (figs. 48-52) we have first two, then more, up to six pairs, of thoracic natatory feet (which in the adult become branchiferous), and traces of as many as nine post-cephalic somites, but the last three never attain appendages. As the young Limulus increases in size, the yolk gradually becomes absorbed, and the larva assumes the position of an Amphipod in the egg, having its dorsal surface convex, instead of its ventral, which is now- concave (fig. 52). There appears to be no stage seen _ in larval Limulus which can be compared with the nauplms stage of A pits (as Dr Packard has supposed). 9 The VI. 82
 * Packard, op. cit. p. 163.