208 



INVERTEBRATE ZOOLOGY 



following a final larval molt the larva has practically attained 

 the adult form. 



Among the parasitic copepods, the nauphus and metanauplius 

 stages are passed within the egg membranes, and the larva at 

 hatching is termed a copepodid because of its fairly close resem- 

 blance to the general organization of the free-living copepod. 



The zoea (Fig. 103 B) is characteristic of the Malacostraca 

 where it is usually the first larval stage but in a few instances it is 

 preceded by a nauplius. It consists of a cephalothorax bearing 



Fig. 95. — Typical stages in the development of the lobster. A, first swimming 

 stage or mysis; B, second larval stage, with abdominal appendages; C, fourth 

 larval stage showing loss of exopodites from walking legs. (After Herrick). 



biramous appendages and an abdomen without appendages. 

 The head bears a pair of lateral compound eyes. Typically, 

 the zoea by several molts transforms into a mysis stage (Fig. 

 95 A), in which even the posterior thoracic appendages are 

 biramous and are of use in swimming. In later development, 

 the exopodite of these biramous thoracic appendages disappears 

 and leaves the unbranched walking legs of the adult. 



In the Brachyura (crabs), the thoracic appendages of the 

 zoea, which are later destined to become the walking legs, develop 

 in the free-swimming larva as only budlike rudiments and never 

 acquire the biramous condition characteristic of the mysis larva. 

 Thus in the crabs the mysis stage has been eliminated and the 



