166 



INTRODUCTION TO ZOOLOGY 



Develop- 

 ment of 

 Crabs. 



Fio. 102.— Series of cast shells obtained from 

 a single individual Sliore Crab (Carciims 

 maenas) kept in an aquarium. The largest 

 shell of these was 2J in. wide. 



reduced. However, it is pos- 

 sible to see them if the abdomen 

 is gently pulled down into 

 the position shown in Fig. 99. 



In the common Spider Crab 

 the front of the carapace, or 

 " forehead," is not rounded as 

 in the shore crabs and swim- 

 ming crabs, but is prolonged 

 in front into a sharply pointed 

 rostrum. The popular name 

 of these creatures is due to 

 the great length of their ''walk- 

 ing legs " in comparison to the 

 size of their body. 



The develop- 

 ment of crabs is 

 specially intei-est- 

 ing. From the eggs hatch 

 out small, transparent, free- 

 swimming larvae (Fig. 100), 

 each with a carapace armed 

 with two long spines ; a pair 

 of large dark eyes is also 

 present. The rudiments of 

 the antennae and the first 

 two pairs of jaw feet are de- 

 veloped, but no appendages 

 farther baclv than these. The 

 long abdomen ends in a forked 

 tail. This larva is known as 

 a zooea larva. 



Gradually the other thoracic 

 appendages develop and also 

 the swimmerets on the abdo- 

 men; then the larva sinks to 

 the ground, looking rather like 

 a little crayfish (Fig. 101) ; this 

 is known as the Megalopa stage. 

 When once it has taken to 

 ground life, and has ceased to 



A\ 



