CRUSTACEA. 335 



All Crustacea show the same direction of development, — i. e., they 

 obey the law of the Articulate type by the commencement of the 

 embryo at its ventral surface, — it is literally built up from below ; 

 and consequently the umbilicus, or the last cicatrix closing in the 

 yolk-sac, is on the back ; the body is completed by the finishing of 

 the segments or rings at the dorsal surface of the body. In regard to 

 the Malacostraca, there are many modifications of the secondary or 

 subsequent phenomena of development. As long ago as 1778, a 

 Dutch naturalist, Slabber*, described and figured a minute swimming 

 Crustacean of the genus called Zoea by modern naturalists ; it was 

 provided with a pair of large and distinct eyes ; its carapace was 

 armed with a long frontal and a dorsal spine ; and its abdomen was 

 terminated by a forked tail. He preserved this little animal alive in 

 sea water, which was daily renewed, and on the fourth day he found 

 that the animal had moulted and changed its form : the feet, eyes, and 

 antennse were more developed, the frontal spine had become compara- 

 tively small, the dorsal one had disappeared, and the tail had changed 

 from the bifurcate to the spatulate shape, and was fringed by a row 

 of short spines. Many years elapsed ere this observation was re- 

 peated, and it seems to have been forgotten, when Dr. Leach, the 

 most accomplished Crustaceologist of his day, founded the principal 

 character of the class Crustacea on the absence of metamorphosis. 



During the spring of 1822, Mr. V. Thompson f, to whom we 

 are indebted for the discovery of the metamorphoses of the Cirripeds, 

 captured an abundance of the singular Zoeas in the harbour of Cove ; 

 the largest of them was daily supplied with fresh sea-water from 

 May 14th until the loth of June, when it died in the act of changing 

 its skin. The disengaged members, invested with the new integu- 

 ments, were changed both in number and form, and corresponded 

 with those of the decapod Crustracea, the anterior pair being 

 furnished with large pincers. Here, therefore, was a strong indica- 

 tion that, under the form of a Zoea, was masked that of some one or 

 other of the higher Crustacea ; and, probably, one of the common 

 species of the Irish coast. To the development of these, therefore. 

 Dr. Thompson next turned his attention, and he succeeded in 

 hatching the ova of the common crab during the month of June, and 

 found that the young were excluded under the form of the Zoea 

 Taumis, with the addition -of lateral spines to the thorax ; where- 

 upon he concluded that the decapod Crustacea indisputably under- 

 went a metamorphosis. 



These observations were, however, called in question, after the 



* CCXXXVI. t CCXXV. No. 1. 1828. 



