HOMOLOGIES AMONG CRUSTACEA. 
109 
frequently happens that animals bearing only slight 
external resemblance to one another in adult life are 
classified together because 
in embryological life they 
show so many resem- 
blances. 
Among the ten thou- 
sand or more species of 
crustacea, there are many 
strange forms which de- 
part from what might be 
called the typical crusta- 
cean structure. Compar- 
ing the common crab 
(Cancer irroratus) with the 
shrimp or crayfish, we 
notice the small size of 
the abdomen folded under 
the flattened carapace. In 
the hermit crab the abdo- 
Fic. 96. — Water-flea (Daphnia 
pulex). 
men is soft and has lost part of the swimmerets, be- 
cause of its habit of using the shell of a snail for pro- 
tection, yet in the young the abdomen of this animal 
Fic. 97.—Cyclops. 
A, dorsal view; 2B, side view. 
is essentially like that of the young crayfish and bears 
homologous parts, 
