CRUSTACEANS. 231 



are seated in front of the head. The caudal extremity terminates in 

 two large leaves, each with two triangular, flat, finny prolongations, and 

 a similar prolongation with the like projections on each side of the two 

 leaves. Besides some appendages below, there are ten limbs provided 

 with claws. Colour grey ; sometimes almost white. 



This animal dwells as a parasite on the outside and inside of the 

 umbrella of some of the larger Meduspe : it often lies supine on its back, 

 curving and closing up Uke the terrestrial Oniscus. It also swims swiftly 

 through the water, where a portion of its shell projects dry and glossy 

 above the surface. 



Twelve or fifteen of these animals being removed from nine Meduste 

 on the 18th of September, several of them appeared to be prolific. The 

 spawn adhered in a cluster high on the abdomen, but specimens remain 

 so closely and pertinaciously in their curvatures, as scarcely to allow 

 distinct inspection. 



A prolific specimen having been committed to a separate vessel on 

 September 19, about fifty young were found there. They were very 

 minute and lively, white to the eye, and grey by the microscope. Their 

 limbs amounted to ten, all of the same character, being provided with 

 claws. The tail folded in like that of a Lobster. 



The young animal, rolling itself up, remains on its back like the 

 parent, a striking feature in the habits of the adult, which continues in 

 this position on any part of the Medusae without falling off, as might be 

 expected, from having no external means of adhesion. 



It was impossible to obtain a view of either the eyes or the caudal 

 extremity of the young. Probably a certain metamorphosis is under- 

 gone, whereby the lower organs are developed. 



Another brood, of about sixty, appeared on September 29, from a 

 prolific specimen in a difierent vessel, aU pure white like the preceding. 



Plate LXII. 



Fig. 6. Asellus meduscE, adult. 

 7. Young, enlarged. 



