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¬ 
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 
Fig. 
96. — Water-flea (Daphnia 
pulex). 
Fig. 97.—Cyclops. A, dorsal view; B , side view. 
is essentially like that of the young crayfish and bears 
homologous parts. 
