78 HISTORY OF CRUSTACEA. CHAP. VIII. 



sedentaria, are produced, according to Pagenstecher, 

 from simple feet of ordinary structure ; and vice versa, 

 the chelae on the penultimate pair of feet of the young 

 Brachyscelus, become converted into simple feet. In 

 the young of the last-mentioned genus the long head is 

 drawn out into a conical point and bears remarkably 

 small eyes ; in course of growth, the latter, as in most 

 of the Hyperinse, attain an enormous size, and almost 

 entirely occupy the head, which then appears sphe- 

 rical, &c. 



The difference of the sexes which, in the Gammarinse 

 is usually expressed chiefly in the structure of the 



siderable change. The difference between the two sexes is considerable ; 

 the females are distinguished by a very broad thorax, and the males 

 (Lestrigonus) by very long antennae, of which the anterior bear an 

 unusual abundance of olfactory filaments. 



Their youngest larvae of course cannot swim ; they are helpless little 

 animals which firmly cling especially to the swimming laminae of their 

 host ; the adult Hyperix, which are not unfrequently met with free in the 

 sea, are, as is well known, the most admirable swimmers in their order. 

 (" II nage avec une rapidite extreme," says Van Beneden of H. Latreillii 

 M.-Edw.) 



The transformation of the Hyperix is evidently to be regarded as 

 acquired and not inherited, that is to say the late appearance of the 

 abdominal appendages and the peculiar structure of the feet in the 

 young are not to be brought into unison with the historical development 

 of the Amphipoda, but to be placed to the account of the parasitic 

 mode of life of the young. 



As in Bracliyscelus, free locomotion has been continued to the adult 

 and not to the young, contrary to the usual method among parasites. 

 Still more remarkable is a similar circumstance in Caligus, among the 

 parasitic Copepoda. The young animal, described by Burmeister as 

 a peculiar genus, Chalimus, lies at anchor upon a fish by means of 

 a cable springing from its forehead, and having its extremity firmly 

 seated in the skin of the fish. When sexual maturity is attained, the 

 cable is cut, and the adult Caligi, which are admirable swimmers, are 

 not unfrequently captured swimming freely in the sea. (See ' Archiv. 

 fur Naturg.' 1852, I. p. 91). 



