TYPE CRUSTACEA. 



385 



or Yasa deferentia {vd), connected witli each organ, and open- 

 ing usually upon the ventral surface of the body at or near 

 the junction of the thoracic and abdominal regions. The 

 origin of these ducts has not yet been discovered, but it has 

 been suggested that they may represent a third pair of ne- 

 phridia. Accessory structures, such as receptacula seminis 

 and cement-glands, for the attachment of the ova in the fe- 

 males, and spermatophore-sacs, in which the spermatozoa a^e 

 encapsuled in spermatophores in the males, are frequently de- 



FlG. 173. — OVAKY AND TESTIS OF ^S«S (after SarS). 



od = oviduct. tb = transverse bar of ovary. 



o« = ovary. ie = testis. 



vd = vas deferens. 



veloped in connection with the ducts, and in the Malacostraca 

 certain of the appendages in the neighborhood of the genital 

 openings are, especially in the males, modified so as to serve 

 as copulatory organs. 



Owing to the great variety of form and structure met 

 with in the various species of Crustacea the group is separa- 

 ble into a large number of subdivisions. Two principal classes 

 are, however, readily discernible, of which the first is 



I. Class Entomostraca. 



In this class the number of segments of which the body is 

 composed varies greatly in the various groups and even in 

 closely-related genera. The abdominal region is in some 

 forms very much abreviated and is destitute of appendages, a 

 rul-e which, however, finds exception in certain Phyllopods in 



