270 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



and turned over to the left, the organs of the pallial 

 cavity can be easily made out. On the extreme left, and 

 arising from the inner surface of the mantle, are two 

 ridges which form the side walls of a groove, the pallial 

 siphon (fig. 8, Siph.). These side walls and the basal 

 part of the groove are continued so as to form a truncated 

 cone with a gutter down one side of it. This pallial 

 siphon lies in the shell siphon, but can be extended a 

 considerable distance when the animal is active. Water 

 entering the mantle cavity passes in by means of this tube, 

 which is characteristic of the carnivorous gastropods. 

 One would imagine from observation of the living animal 

 that the siphon was connected with some important system 

 of sense organs. It is continually in motion from side to 

 side, and extends much further from the animal and is 

 more active than the tentacles. 



Examination of the mantle cavity in this light reveals 

 an interesting series of organs. The osphradium, which 

 is a darkly pigmented structure on the left side, is 

 situated right across the end of the siphon (fig. 8, Osph.). 

 Thus all water entering the pallial cavity must pass over 

 it before reaching the other organs. 



The osphradium is a narrow organ composed of two 

 series of leaflets arranged along the sides of a central axis. 

 It will be discussed further in the chapter on the sense 

 organs. To the right of the osphradium is the gill 

 (fig. 8, Ct.). It is separated from the osphradium by the 

 ctenidial axis which can be seen as a white ridge running 

 from the most distal part of the gill to the inner end of 

 the pallial cavity. The gill itself is composed of leaflets 

 arranged on one side of this axis only, the topographical 

 right. Between the ctenidium and the cut side of the 

 mantle the inner surface of the latter is occupied by the 

 large mucous or Hypobranchial gland (fig. 8, Mu. gl.), 



