222 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



Dichogaster by supposing that the latter genus has descended 

 from the former through a process of reduction of the pros- 

 tates and through a displacement of the male pore. That 

 such reduction and displacement has actually taken place is 

 evident from the fact that some of the species now described 

 offer characteristics which are intermediate between the 

 two genera. A perfect series is had from the typical Ben- 

 hamia with its male pores in the equator of XVIII to the 

 typical Dichogaster with its male pores opening with the 

 prostates in the equator of XVII . 



Benhamia viridis forms the first link in this chain. In 

 this species the pore of the sperm-ducts has advanced more 

 than one quarter of a somite, being found in the anterior 

 part of XVIII near the intersegmental groove. The next 

 link is seen in Dichogaster Ribaucourti, where we find the 

 pore of the sperm-ducts in the intersegmental groove 

 between XVII and XVIII. In this species the prostates 

 have remained. The next link is Dichogaster Townsendi, 

 in which the pore of the sperm-ducts is in the equator of 

 XVII, together with the prostate pore, but the remains of 

 the former spermiducal somite are yet recognizable in an 

 accessory septum found in somite XVII. This accessory 

 septum is situated immediately posterior to the sperm-ducts, 

 lying between them and the septum separating somite XVII 

 from the fully developed somite next posteriorly. As might 

 be expected, somite XVII is slightly larger than either of 

 the adjoining somites, but otherwise there is no exterior 

 sign that two somites have been fused together. 



The development of Dichogaster from its B enhamia-Xik.^ 

 ancestors has thus comprised two distinct processes: one 

 consisting of a displacement of the male pore forward, the 

 other in a reduction in size and the disappearance in part of 

 the male pore-bearing somite, XVIII. 



As regards the reduction of the prostates, we find this to 

 be more or less complete and not subjected to any general 

 rule. While in some species the prostates have disappeared 

 from the somites posterior to the male pores, in others they 



