226 



AN INTRODUCTION TO ZOOLOGY 



FIG. 1 20. Peripatus capensis. (From Shipley and MacBride after Sedgwick.) 



with the exception of Peripatus, are not segmentally arranged; 

 Annelids have a well developed ccelom; 

 in arthropods the ccelom is restricted 

 to the cavities of the excretory and 

 reproductive organs. 



The Crustacea are divided into two 

 subclasses, the Entomostraca and the 

 Malacostraca. The Entomostraca in- 

 cludes most of the small simple species 

 (Fig. 119). These have a variable 

 number of segments; no gastric mill 

 in the stomach; and in many species 

 hatch as a larval form called a Nauplius 

 (Fig. 126, A). The Malacostraca are 

 usually large. They have a definite 

 number of segments five in the head, 

 eight in the thorax, and six in the 

 abdomen; the stomach contains a gas- 

 tric mill; and the Nauplius stage is 

 usually passed through within the egg 

 before hatching. The crayfish belongs 

 in this subclass. 



Class Onychophora contains only a 

 few annelid-like animals, the best known 



FIG. 121. Dorsal view of a being Peripatus capensis (Fig. 120). 

 centipede. /, antenna; Segmentally arranged nephridia, stump- 

 2, poison claw. (From like legs, simple eyes, and tracheae are 

 Shipley and Mac- some o f ^ Q organs possessed by mem- 

 bers of this class. 



Bride.) 



