16 MEMOIR II. 



imates much to that of the Shrimps and Prawns, but 

 independent of the number and structure of the feet and 

 branchia above stated, the abdomen, which is always kept 

 extended, is furnished with fins of a very peculiar struc- 

 ture, Plate I. fig-. 9, added to which, the female is provided 

 with a post- thoracic pouch, Plate I. fig. 1. u. composed of 

 four concave valves, which are articulated inside the base 

 of the two posterior pair of legs, and strongly ciliate or 

 pectinate where they meet in front : of these valves the 

 posterior are the most capacious, and exterior to the 

 others ; it is within this pouch, that the eggs are received 

 when excluded from the ovarium, and enveloped in a 

 mucous or subgelatinous secretion, and gradually developed 

 without any visible attachment to the parent. The ova 

 when first received into the pouch, are considerably more 

 advanced than those of the Shrimps, Crabs, &c. on their 

 first expulsion, and by no means so numerous, a circum- 

 stance more than compensated by the rapidity with which 

 one brood succeeds to another during the whole of the 

 spring and summer months : the number of broods produ- 

 ced by one individual, as well as the time occupied in their 

 evolution, have not been determined, but the changes 

 which the embryo vindergoes in configuration are suffi- 

 ciently obvious ; in the present instance, these cannot be 

 considered as metamorphoses, but simply a gradual deve- 

 lopement of parts, hence the Shizopoda may be regarded 

 as one exception to the Crustacea undergoing transforma- 

 tions, another character by which they are separable from 

 the true Shrimps, &c. to which they bear the same relation 

 that the Syren among the reptiles (amphibia) does to the 

 family of Lacerta. The first change which is perceptible 

 in the ova after their reception into the maternal pouch, 

 is a slight elongation at one end, and the appearance of 

 two short members at each side, Plate I. fig. 10, b ; this 

 elongation which proves to be the tail, increasing in length, 

 shortly after, becomes forked at the end^ accompanied by 



