When Astacus quits the egg it only differs from the adult in certain points summarised here from Professor Huxley's account. The cephalothorax is relatively large and convex in shape; the short rostrum is bent down between the eyes; the thoracic sterna are relatively wide; the chelae of the forceps are slender, and the tips of all the chelae are strongly incurved, the young Crayfish attaching itself by those of the forceps to the empty egg-case: the dactylopodites of the two last pairs of thoracic limbs are hooklike: the first pair of abdominal appendages is undeveloped: the sixth is included within the telson, which is a simple broad oval plate usually notched in the middle of its hinder margin. Setae are few in number and are mostly uncalcified prolongations of the cuticle not sunk in pits, and devoid of barbs. It is noteworthy that at the stage of development in which the first and second antennae and mandibles are present as rudiments, and which therefore corresponds to the Nauplius of other Crustaceans, a cuticle is formed and then moulted. A similar phenomenon occurs in other cases, e. g. in Nebalia, in Mysis within the incubatory pouch of the mother, and has been noted in the Isopoda, e. g.
Asellus. It is perhaps a general phenomenon in Crustacea with a shortened developmental history. The majority of Decapoda differ from Astacus in having a well-marked metamorphosis. The prawn Penaeus is hatched as a Nauplius, most other Decapoda as a Zoaea. The Lobster (Homarus), placed by M. Milne Edwards but not by Faxon among the Astacidae, starts in the Mysis stage, i. e. with the thoracic feet biramose and natatory. One or two fresh water Decapoda have a shortened metamorphosis: one or two Land Crabs none at all.
Much has been written as to the ancestral character of the Nauplius and Zoaea. The subject will be found discussed in Balfour's Comparative Embryology, i. p. 417, to which must be added some recent remarks of Claus in his Beitrage, etc, Arb. Zool. Inst. Wien. vi. 1885, p. 91. It is at present impossible to say how the Nauplius has been derived, and what are its affinities. It is apparently not a simple organism, for in the Copepoda Natantia its body is divided into three somites, but the segmentation disappears before hatching. Of its three pairs of appendages the first is uniramose and for the most part sensory in function, the second and third are biramose and natatory. The three pairs correspond to the first and second antennae and mandibles of the adult.
It is generally agreed that the second antennae are post-oral appendages. They are innervated from a post-oral ganglion in the Nauplius, and in the adult Apus, Limnetis, Branchipus among Phyllopoda. Daphnia much resembles Branchipus. It may be added that in the Nauplius they lie at the sides of the mouth, and as a rule develope a masticatory hook.
The first antennae on the contrary are generally held to be prae-oral appendages. Claus in his Beitrage (supra) points out (1) that they retain their uniramose character in Entomostraca, whilst in Malacostraca they become in most instances secondarily biramose; (2) that they are sensory in function; (3) that they arise from a region of the head morphologically unlike (as in Chaetopoda) the somites of the body. He appears to regard them (1) as limbs, and (2) at the same time to compare them with the prae-oral tentacles of Chaetopoda and the antennae of Myria-poda and Insecta. As to this second point however it may be remarked that they do not originate from the procephalic lobes as do the antennae of Myriapoda and Insecta.
Ray Lankester first drew attention to the innervation of the first and second antennae in Apus (Q. J. M. xxi. 1881), and Pelseneer (Q. J. M. xxv. 1885) has investigated the nervous structures microscopically. He finds that the ganglia of the first antennae are contained in the supra-oesophageal ganglion, but are separate from the mass of ganglion cells supplying the eyes. They are connected by a transverse commissure. The nerve to the first antenna runs backwards accompanying the oesophageal commissure for some distance, as it does in Limnetis. He says that in Branchipus and Daphnia the corresponding nerve arises from a group of cells distinct from the rest of the supra-oesophageal ganglion. It may be observed that Rathke states that in the embryo Crayfish first and second antennae alike are supplied from a ganglionic rudiment distinct from an anterior rudiment apparently.
The natural conclusion is that the supra-oesophageal ganglion consists of two distinct parts in Apus, etc, i.e. of a true prae-oral ganglion supplying the eyes, and a second pair of ganglia shifted forwards supplying the first antennae. The shift forwards explains the backward course of the nerves. Whilst in Apus, etc, the second antennae have a post-orally placed ganglion, in higher Crustacea this ganglion also has shifted forwards; but there is evidence to show (embryo Astacus) that there is a distinct ganglion, supplying in this instance both first and second antennae, which fuses with an anterior rudiment to form the supra-oesophageal ganglion of the adult. It seems probable therefore that the first antennae, like the second, are in reality primitively post-oral appendages, or at any rate are homologous with the limbs borne by the post-oral somites of the body.
The Arachnida afford us an instance of a group of Arthropoda in which all the appendages are embryonically post-oral. The first pair of appendages however is invariably shifted in front of the mouth during growth. In Scorpio and Limulus at least their nerves are said not to come from the supra-oesophageal mass but from the commissures. The ganglia however have not been investigated microscopically. It is possible that these animals retain an archi-cerebrum, i.e. a supra-oesophageal ganglion not fused with other ganglia placed posteriorly to it. The Crustacea, on the contrary, evidently possess a syncerebrum, i. e. a supra-oesophageal ganglion fused with one or two posteriorly placed ganglia.