40 PHYLUM ARTHROPODA 



Class : Crustacea. Division : Malacostraca. Order : Decapoda 



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 zoea 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 

 zoea 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 the Atlantic 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, that is, the maxillipeds, which, 

 however, are used for locomotion. The remaining thoracic 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 1. Draw a side view of a zoea on a large scale, representing 

 accurately the appendages, and label the parts observed. 



Mount a megalopa and study it under the microscope. You will 

 observe that it is much larger than the zoea, that it has acquired 

 a relatively much larger cephalothorax and abdominal append- 

 ages, and is much more crablike than the zoea. But it still has 

 a long abdomen, and at the end of this is a swimming fin. The 

 megalopa is a swimming animal, like the adult lobster, but it is 

 gradually assuming the characters of the adult crab. Its two 

 anterior maxillipeds have lost their locomotory character, which 



