574 



METAMORPHOSIS OP CRUSTACEA. 



748. In the class of Crustacea there is great variety as 

 to the degree of metamorphosis undergone by the young after 

 their emersion from the egg ; for whilst there are several in- 

 stances in which either the mature form, or one closely re- 

 sembling it, is presented from the first, the more common fact 

 is that the early or larval condition is extremely dissimilar to 

 that of the parent, and that a succession of changes has to 

 be gone-through before the latter is attained. This is in no 

 instance more remarkable than in that of the common Crab, 

 whose larva (fig. 48) was long known under the name of 

 Zoea, and was supposed to belong to a type altogether dif- 

 ferent. No instance of agamic reproduction by eggs is 

 as yet known among the higher Crustacea ; but in the 

 Entomostracous division there are probably many examples 

 of it. Thus in the little Daphnia (ZooL. 879), one of the 

 commonest of the " water fleas," the ordinary eggs seem to be 

 always " agamic ; " whilst the eggs which are formed within 

 the peculiar case termed the epkippium, and which seem 

 enabled by its protection to endure a degree of cold that is 

 fatal to the ordinary eggs as well as to the parents, are the 

 products of sexual action. The Cyclops, again, has been 

 found, like the Aphis, to produce many 

 successive broods, which broods repeat the 

 like mode of propagation, without the appear- 

 ance of a male. Among the examples pre- 

 sented by this class, of entire change of form 

 in the progress of development, none are 

 more remarkable than those which are met 

 with among the suctorial tribes, which live 

 as parasites upon the exterior of other 

 animals, especially Fishes. As an example 

 of this change we may refer to the Lerncea 

 (fig. 312), an animal which is not unfre- 

 quently found clinging to the eyes and gills 

 of fish, the anterior part of its body being 

 commonly imbedded in the substance of the 

 part to which it attaches itself. This creature 

 is characterized by the size of its large suc- 

 torial trunk a, and by that of its single pair 

 of legs c, which terminate together in the sucker/; and by the 

 immense development of the abdominal portion of its body d, 



Fig. 312. LERNJEA. 



