Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/679

Rh C K U S T A C E A 645 Eurynomc, and an allied genus to Achccus, the spines are wanting in the first two genera, and but of inconsiderable size in the last- named genus. &quot; The following are the more important peculiarities in the zoea of the crabs, although less striking than these processes of the carapace, which, in combination with the large eyes, often give them so singular an appearance. The anterior (inner) antenna} are simple, not jointed, and bear two or three olfactory filaments at the extremity ; the outer antenna? frequently form a long spine-like process, and bear a minute squamiform process like the antennal scale in prawns. Of the natatory feet (afterwards maxillipeds) only two pairs are present, the third is entirely wanting, or present, like the five following pairs of feet, only as minute buds. The tail in zoea is very variable in form, but nearly always bears three pairs of setae upon its hinder margin &quot; x When the young zoea first escapes from the egg, it is enveloped by a membrane veiling the spinous processes of the carapace, the seta? of the feet, and the antenna. ; but this is cast off in a few hours. The zoeie of Porcellanci (fig. 28) seem to differ widely from true crabs, but really approach them very closely. The dorsal spine is wanting, but the frontal and lateral spines are of extraordinary length, and directed straight forward arid backward. The tail bears five pairs of setae. The zoea of Hippo, erennitn also resembles that of the crab in general appearance and in mode of locomotion (fig. 29). The cam- pace has only a short broad frontal process ; and the caudal plate is edged with numerous short setae. The zoeae of hermit-crabs (fig. 30) have simple antennules like those in the Brachyuran zoeae. The antennae bear a scale-like appendage on the outside analogous to that in the prawn. There are only two pairs of well-developed natatory feet (maxillipeds), but the third pair r.re present in the form of two-jointed rudimentary appendages destitute of setae. The hinder border of the tail bears five pairs of setae. The zoe ai of the shrimps and prawns agree closely with the Anomoura. They have a small median eye between the large com pound ones. The third pair of maxillipeds are always present. 2 In investigating the development of the spiny lobster, Glaus found embryos in the ova with completely segmented bodies, but wanting the abdominal and caudal appendages and the last two thoracic somites. They have a single median eye, the anterior antennas are simple, the posterior have a small secondary branch ; the maxillipeds are divided into two branches. 3 The most singular example of lowly development recorded by Fritz Muller is that of a prawn of the genus Penceus. FIG. 31. Nauplius of a Prawn. Magn. 45 diam. (Fritz Muller.) The young appear to quit the egg with an unsegmented ovate body, a median frontal eye, and three pairs of natatory feet, of which the anterior pair are simple and the others biramose, agreeing with the larval form common to the 1 Fritz Muller, Facts and Arguments for Darwin, pp. 49-52. 8 Coste asserts that he has bred young Phyllosoma from the ova of Palinurus vulgaris, a statement, says Fritz Miiller, that requires further proof, especially as the more recent investigations of Claus upon Phyllosoma by no means favour this conclusion (Fritz Muller, op. cit. P. 57). 32. Young Zoea of the same Prawn. 45 diam. (Fritz Miiller.) Magn. lower Crustacea, to which 0. F. Muller has given the name of nauplius.^ In this stage there is no trace of a carapace, no trace of paired eyes, no trace of masticating organs, and the mouth itself is overarched by a helmet-like hood. In one of these species the interme diate forms which lead from the naup lius to the prawn have been discover ed by Fritz Muller in a nearly continu ous series (op. cit. p. 57) After successive moults the nauplius gives place to the zoea period, during which the paired eyes, the segments of the thorax and abdomen, and the various appendages are produced in bud- like succession. The zoea next passes into antennae cease to serve for locomotory organs, and their place is taken by the thoracic feet clothed with setse (fig. 34). The abdomen, furnished with powerful muscles, jerks the animal through the water in a series of lively jumps. In the case of those Crustacea in which the young, as in Mysis, are retained within the incubatory pouch of the parent after quitting the egg, the larva emerges from the egg in a far more rudi mentary and destitute con dition than in those genera in which no such protective arrangement exists. Van Bemden, whose description of the development of Mi/sis is confirmed by Fritz Muller, mentions the very curious fact that the first segment that makes its appearance is the tail. la other stalk-eyed Crustacea the embryo has the ventral surface of the anterior and posterior halves of the body folded together, and the dorsal surface forms the external convexity of tl a young animal within the egg ; but in Mysis the ventral surface is external and convex. The tail soon acquires the furcate form characteristic of the zoea of the prawn ; two thick ensiform appendages next make thair appearance at the anterior end of the body, and behind these a pair of 4 Compare fig. 31 with the nauplii of A pus and Artemia, 5 and 6 of fig. 57, and with that of Balanus, fig. 60, A, and fig. 61. io. 33. Youngest observed Zoga of another Prawn. The minute buds of a third pair of maxil.ipfcds are visible. The formation of the abdominal segments has commenced. Paired eye still want- ing. Magn. 45 diain. (Fritz Muller.)
 * Fritz Miiller, op. cit., pp. 53-55.