GENERAL ZOOLOGY 



Artery 



Large 



ganglion 



cells 



Layer of retinae 



Subocular space 

 j" Optic ganglion I 



Optic ganglion II 



■tO, 





ganglion IK".'' ' \" ' ,' ' '>'t^ fnli/" 'll 



cells l-f'W ' P ''','•, '-liW' /mi 



Adductor hSA m /'■ ['I' ', /i] "^ f\ 



muscle / > '. ' •/. ■ < ' ,') I , ■' V * U^ 



"Optic ganglion III 



"Optic ganglion IV 



"Abductor muscle 

 "Optic nerve 



receptacle of the female. This is a pouch-Hke cavity lying between the bases 

 of the fourth pair of pereiopods. The seminal receptacle has no internal 

 opening; it serves merely to hold spermatozoa received at copulation until 

 the subsequent time of oviposition. The large-yolked eggs, released from the 

 oviducts, pass backward between the bases of the pereiopods and are fertilized 

 by spermatozoa issuing from the seminal receptacle. The resulting zygotes 

 are then attached to bristles on the swimmerets of the female. After hatching, 

 the larvae remain attached to the swimmerets and develop through a series of 

 molts until they have reached the stage of miniature adults and are able to 

 begin an independent life. 



Other Crustacea. According to older classifications, the class Crustacea 

 was divided into only two subclasses, the Entomostraca and the Malacostraca. 

 The Malacostraca, to which the crayfish, lobster, and other highly developed 

 crustaceans belong, appears to be a natural grouping with distinctive 

 characteristics. Thus, all the Malacostraca have a six- or eight-segmented 

 abdomen, an eight-segmented thorax, abdominal appendages, and a gastric 

 mill. In contrast, the Entomostraca of older authors constitute a hetero- 

 geneous assortment of crustacean types, many of them differing from one 

 another as greatly as they differ from the malacostracans. We shall use the 

 term Entomostraca descriptively, to refer to crustaceans which are small, 



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