766 PROF. a. C. BOUKNB ON THE 



is laid on the fact that the internal partitions of the shell are 

 absorbed in both these families. This absorption has not pro- 

 ceeded quite so far in the Neritidse as in the Helicinidse. In 

 Nerita and Paranerita there is a recess in the upper right-hand 

 part of the shell which contains that lobe of the visceral mass which 

 consists wholly of liver and gonads and represents the visceral 

 spire of other Crastropods. This recess and the lobe of the 

 visceral sac corresponding to it are not found in the Helicinidfc : 

 the wall of the oesophageal division of the stomach comes very- 

 near to the surface (PI. XXXIII. figs. 17 & 18), and the liver and 

 gonads are disposed at the sides of and above the pyloric division 

 of the stomach. The more coiled appearance of the whole is 

 due to the elongation of the post-tentacular and dorsal regions, 

 not to the retention of a larger section of the visceral spire of the 

 presumed gastropod ancestor than in the Neritidse. 



From what precedes, it follows that most of the peculiarities of 

 the Helicinid organization are the result of excessive growth and 

 elongation of a particular region of the body, and it is an 

 interesting confirmation of the correctness of the above account 

 of the manner in which the Helicinid organization has been 

 derived from the Neritid that, if one makes a plasticine model 

 of the stomach, kidney, rectum, and intestinal coils as they occur 

 in Paranerita, and then rotates the stomach in the manner 

 described, the intestinal coils assume very nearly the position 

 foimd, with more or less variation in detail, in all Helicinids. 



After this general explanation of the mutual relations of the 

 principal viscei-al organs in the Helicinidae, I need only refer to 

 particular features in the several systems of organs which I have 

 to describe in detail. Before proceeding, it should be put on 

 record that there is not a rudiment of the ctenidium in the 

 Helicinidse, and I cannot even find a trace of an osphradium. 

 The cephalic penis, characteristic of the males of the Neritidpe, is 

 also absent, and there is no external diiference between the males 

 and females in any of the species that I have examined. 



The Alimentary Tract. 



The complex of organs formed by the buccal cavity, the pharynx, 

 the oesophagus with its smaller and larger glandular annexes, the 

 radula, the radular sac, and the odontophoral cartilages and their 

 muscles, can only be studied by dissection, and this is by no means 

 an easy task in animals so small as most species of Helicinidae are. 

 The relations of the various organs to one another are far too 

 complicated to trace out in sections. The following description 

 applies chiefly to Alcadia palliata and A. hollandi, but will serve 

 almost equally Avell for any of the other species that I examined, 

 for all are very much alike except for the details of the radular 

 teeth. 



The mouth is a gaping circular orifice, situated at the extremity 

 of the downturned snout: it is surrounded by folded muscular lips. 



