172 DESCRIPTIONS OF PREPARATIONS. 



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 Nefialia, 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 de- 

 velopmental 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. 41 7^ 

 to which must be added some recent remarks of Claus in his Beitrage, &c., 

 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 (i) that they retain their uniramose cha- 

 racter 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 (i) 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 in- 

 vestigated 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 accompany- 

 ing 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 



