662 THE C.l'.M-'.RATlVE ORGANS. 



The testes of many insects consist of several or numerous 

 follicles, which either open separately into the vas efferens or 

 unite to form a common cavity ; sometimes, as in the Lepi- 

 doptera, the two testes are fused into a single median organ, 

 but there are always two ducts, which often have transverse 

 inter-communications. 



The Duct of the Testis, Vas Efferens. It is convenient to dis- 

 tinguish the paired and azygos ducts of the testes by distinct 

 names ; authors usually term both ' vasa deferentia,' I have 

 preferred to use the term 'vas efferens' for the duct of the 

 testis, and 'vas deferens' for the azygos duct formed by the 

 union of the ducts of the testes and of the so-called vesicuhe 

 seminales, or paragonia. 



In the Blow-fly the vas efferens is a thick walled tube about 

 0*7 mm. in length. It has an external diameter of '05 mm., but 

 its lumen is less than half this diameter; its walls consist of 

 small columnar cells ; the outer surface is covered by a thin 

 peritoneal layer continuous with the adipose tissue which 

 covers the capsule of the testis. 



The Paragonia (Fig. 90, pg), the so-called vesiculae semi- 

 nales of authors, are a pair of large sacs 1*5 mm. in length and 

 3 mm. in diameter, which open with the vasa efferentia into 

 the vas deferens. The wall of the sac consists of a single layer 

 of long columnar cells covered externally by a thin layer of 

 peritoneal tissue. The sac is filled with a granular, highly 

 coagulable milky fluid. 



The paragonia of many insects are much convoluted, and in 

 others exhibit numerous follicles. The term vesicular semi- 

 nales is certainly misleading, as they never, so far as I know, 

 contain any seminal filaments. The Cockroach, Periplaneta, is 

 an apparent exception, but I think that the real paragonia in 

 this insect never contain seminal filaments. 



The relation of the Paragonia and Testes. It is well known 

 that in Periplaneta the vas deferens is short and wide and 

 terminates in front in a large number of caeca! gland tubules, 

 which constitute the so-called mushroom-like gland of Huxley. 

 These sacculi are of two kinds, and have been distinguished as 



