574 METAMORPHOSIS OF 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 (Zoon. § 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 ephippium, and which seem 
enabled by its protection to endure a degree of cold that is 
fatal to the ordinary eggs as wellas 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 Lernea 
(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 
Fig. 312.—Lervma. 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, 
