and on Six New Species of Criitftacca^ ^c» 181 



peavance of two small segments arising from the posterior 

 part of the body. The larva, a short time after being- hatch- 

 ed, becomes considerably elongated ; it has got three pair of 

 legs, each bearing long spines at the extremity. The third 

 pair were making their appearance before the animal had left 

 the ovum. There are also two spines on each side of the 

 body, at its posterior extremity. The difficulty of keeping 

 these animals alive for any length of time in confinement, 

 has prevented me tracing their further development. 



SIJCTION III. 



Oil Zoe — 'The development of the Ovumj and the meto/morphoses 

 of Carcinus Mcenas, and Pagurus Bernhardus. 



The absence of a metamorphosis in the Crustacea, was con- 

 sidered by Leach to be one of the most decisive characters of 

 the class. This opinion, however, has been overturned by 

 J. V. Thompson, who, by a series of observations, has dis- 

 covered that the Crustacea do undergo a metamorphosis, and 

 that what have hitherto been considered as perfect animals, 

 viz.— the Zoe of authors, are only the larva of different spe- 

 cies of Crustacea Decapoda. Mr Westwood again has assert- 

 ed, that the Crustacea do not undergo any metamorphosis, 

 bringing forward at the same time observations, the results of 

 which were opposed to those of Mr Thompson. Being 

 anxious to observe for myself, I obtained the different species 

 of the higher Crustacea, common on the shores in this neigh- 

 bourhood. When loaded with spawn, they were kept alive 

 in vessels of salt water, and in every case I have found the 

 ova to produce Zoe ; and even all the lower Crustacea which 

 I have yet examined undergo a metamorphosis, although not 

 so decided as that of the higher Crustacea. At present I 

 shall describe the Zoe of two of our most common species, 

 viz. — Carcinus Mcenas and Pagurus Bernhardus. 



When the larva) of the common crab are newly hatched, 

 the body is destitute of spines, but, in a short time, both the 

 dorsal and frontal spines make their appearance (fig. 18, PI. III.) 

 The eyes at this time are very large, the central or reticulat- 

 ed portion black, with a beautiful shade of green. The fron- 

 tal spine as long as the carapace, sharp and pointed. The 



