78 SEA-SHORE LIFE 
hard skin-armor of the body affords excellent attachment for the 
muscles, and it is well known that in proportion to their size in- 
sects and crustaceans are the strongest of all animals. It has even 
been calculated that if a man possessed muscles as strong in propor- 
tion as those of a flea he could readily leap over St. Paul’s Cathedral. 

Fig. 46; YOUNG ROCK-CRAB. From life, natural size. Showing the rapid enlargement 
that took place after the shell was moulted. The Crab had lost one of its nippers, 
but this was regenerated after the moult. 
Being encased in a natural armor crustaceans can not grow at 
a uniform rate, but enlarge suddenly at the periods when the shell 
is shed. This occurs at fairly regular intervals, and the entire 
shell is shed, even the coverings of the eyes and part of the lining 
of the stomach being cast off. The creature is then soft and helpless, 
and usually remains hidden in some safe retreat until the body has 
expanded and the new shell hardened. 
The appendages of crustaceans are of various sorts such as 
feelers, mouth parts, claws, legs, egg-carrying organs, swimming and 
breathing organs, and stalked eyes. 
The Crustacea are divided into two sub-classes, the lowest 
called the Wntomostraca, the barnacles and water fleas, have a var- 
lable number of body segments, and the appendages are usually 
forked, and are apt to be quite similar each to each. The higher 
sub-class called the Malacostraca includes the crabs, lobsters, 
shrimps, and sand fleas. Their bodies consist of twenty segments; 
five in the head, eight in the middle part of the body, and seven in 
the abdomen; and their appendages are apt to be dissimilar each 
from each. 
