CRUSTACEA 



LARVAL DECAPODS. THE ZOEA OF THE CRAB. THE MEGALOPA OF THE 

 CRAB. THE MYSIS STAGE OF THE LOBSTER 



These names have been given to certain larval forms of the 

 crab and the lobster, as well as to those of other of the higher 

 crustaceans. It is as zoeae that the crab and the higher crusta- 

 ceans generally leave the egg. The zoe'a of the crab grows 

 into the megalopa, which in time grows into the adult animal. 

 The stage in which the lobster is born is more advanced than 

 the zoe'a and is called the mysis stage. All of these larvae are 

 minute animals and are more or less common in the surface 

 waters of the sea along our coast. 



Mount several zoeae of the crab on a slide and study them 

 under the microscope. The body will be seen to be divided into 

 two body-divisions, a cephalothorax and an abdomen. The former 

 is covered with a delicate carapace, from which project one or 

 more spines. When the animal is newly born it possesses the 

 typical five pairs of cephalic appendages, and the anterior two 

 or three pairs of thoracic appendages, i.e., the maxillipeds, 

 which, however, are used for locomotion. The remaining tho- 

 racic and the abdominal appendages are wanting, but appear as 

 the animal increases in size, those anteriorly situated appearing 

 first. The animal has two stalked eyes. 



Exercise i. Draw a side view of a zoe'a on a large scale, repre- 

 senting accurately the appendages, and label the parts 

 observed. 



Mount a megalopa and study it under the microscope. We 

 observe that it is much larger than the zoe'a, that it has acquired 



