TTPE MOLLUSC A. 



303 



duct {vd), having apparently no relation to the nephridia and 

 opening into the mantle-cavity to the right side of the anus. 

 In the more primitive Gasteropods, however, such as Haliotis, 

 Fissurella, and Patella, the nephridia, as in the Solenogastres, 

 serve as reproductive ducts ; and it has been suggested that 

 the special reproductive duct of the remaining Gasteropods 

 may represent the left nephridium, which is usually described 

 as having disappeared. The reproductive duct, especially in 

 hermaphrodite forms, has developed in connection with it 

 accessory glandular structures as well as external copulatory 

 organs, the whole reproductive system becoming highly com- 

 plicated. An account of the more important arrangements 

 will be more satisfactorily given in connection with the 

 various orders. 



1. Order Prosobranchia. 



The Prosobranchia are, with very few exceptions, marine 

 Gasteropods, provided with well-developed shells, which are 

 usually spirally coiled, the height of 

 the spiral varying, however, in different 

 forms. In some, such as Patella and 

 Fissurella, the shell has a simple 

 conical form, without any indication 

 of a spiral ; and since these forms in 

 many respects show primitive charac- 

 ters, it might be supposed that this 

 type of shell was also primitive. These 

 very forms, however, show also that 

 asymmetry of parts, which is character- Fig.135.— Shells of Proso- 

 istic for the Gasteropods, and which bbanch Gasteropods. 

 accompanies the rotation of the mantle- 

 cavity, and furthermore, in Fissurella 

 at least, a distinct indication of a 

 spiral coiling, is present in the shells of 



young animals. It seems more probable, accordingly, that 

 these conical shells are to be regarded as secondary modifi- 

 cations of an originally spirally-coiled shell. 



The mantle-cavity is situated in front of the well-devel- 

 oped visceral hump, and is usually somewhat capacious, com- 



A, AcTnma testudinalis 

 (after Gould) ; B, Haliotis 

 (after Lehnis); C, Turritella 

 (after Leunis). 



