LARVAL DECAPODS 51 
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 zoée that the crab and the higher crusta- 
ceans generally leave the egg. The zoéa 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 zoéa and is called the mysis stage. All of these larve are 
minute animals and are more or less common in the surface 
waters of the sea along our coast. 
Mount several zoée 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, «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 1. Draw a side view of a zoéa 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 zoéa, that it has acquired 
