Thompson's Zoological Researches. 2A9 



fying the statements of Slabber, it was daily supplied with fresh sea- water 

 during a month, at the expiration of which time it died while in the act of 

 changing its skin, and of passing into a new form, entirely different from 

 that which it previously exhibited, as was evidently shown by its dis- 

 engaged members, which were changed in number as well as shape, and 

 corresponded with those of the Decapoda, consisting of five pairs, the 

 anterior of which was chelate. The metamorphosis not having been 

 completed, no knowledge of its general form could be obtained, and a 

 second experiment subsequently made, failed equally in determining this 

 point; the animal in this instance also dying while imdergoing the 

 change, the result of which was perfectly similar ; the members from 

 being natatory and deeply cleft, having become simple and adapted for 

 crawling only. 



A further evidence of the existence of metamorphosis among the 

 Crustacea is afforded by the fact that Mr. Thompson has succeeded in 

 hatching the ova of the common Crab, f Cancer Pagurits,J the young of 

 which were found to be similar in form to the Zoea Taurus, Hence he 

 concludes, perhaps too universally from such Hmited observations, that the 

 Decapoda generally undergo metamorphosis, being in the first stage of 

 their existence essentially natatory, and the greater number of them be- 

 coming afterwards, in their perfect state, incapable of swimming, and 

 being then furnished with cheltB, and with feet almost solely adapted for 

 crawling. 



That the whole of the Crustacea, however, are not subject to metamor- 

 phosis, is shown by the second memoir, " On the genus Mysis, or 

 " Opossum Shrimp," in which the history of one of the species is traced 

 ah ova. From this it is evident that the only changes which take place 

 in the animals of this group consist in the successive and gradual de- 

 velopement of parts. During their growth the young are contained in a 

 pouch, composed of four concave valves, which is attached to the pos- 

 terior part of the under surface of the thorax of the female. An analogous 

 structure, it may be remarked, is observable in many of the Crustaces 

 Heterobranches of Lamarck, of which the Onisci and Gammari aflford 

 famihar instances. 



The descriptions of Zoea and of MysiSy and of all the known species 

 ,of the latter, are given at considerable length, and their habits, so far ^ 



Vol. IV. m 



