PARASITIC CRUSTACEA. 207 
from independent predatory genera to others occasion- 
ally parasitic, and again from these to others which 
on leaving the egg immediately attach themselves for 
life. Here, as elsewhere, parasitism seems an adaptation 
to new habitats, which is recorded in the biography of 
the individual with a reminiscence of the previous form. 
The circumstances of the parasitic worms are repeated 
by the parasitic Crustacea, as, moreover, a probably 
primordial form of the crab family is preserved in 
the metamorphoses of several orders of this large and 
diversified, though coherent class. The larva, which, 
it may safely be assumed, 
approximates closely to 
the primordial form, was 
at one time taken for an 
independent genus and re- 
ceived the name of Nau- 
plius. Hence a Nauplius 
phase is spoken of, which 
obtains especially among 
the lower Crustacea, the 
Copepoda, parasitical Crus- 
tacea and Cirripedes, and 
the remarkable Rhizopoda connected with them; but 
is not wanting in the highest order, the decapodous 
stalk-eyed crab. We shall later have to make acquaint- 
ance with the so-called curtailed development which 
among the crabs has been adopted by the decapods, and 
it was formerly supposed by all. Were this actually 
the case, we should still, by analogy, infer their connec- 
tion with the other orders repeating the Nauplius phase 
in the course of their development; but it was a welcome 
10 
FIG, 16. 
