GASTEROPODA. 347 



The sexes are separate ; a single reproductive organ extends dorsally 

 between gut and intestine almost the whole length of the body ; the 

 genital ducts are paired and open posteriorly in front of the excretory 

 apertures. The ova with chitinous spiny shells are usually retained for 

 some time by the female between the mantle and the gills. The 

 segmentation is holoblastic, and a gastrula is formed by invagination. 



2nd Order of AMPHINEURA, APLACOPHORA, e.g., Neonienia, 

 Proneomenia, and Ch&toderma. 



The members of this order are worm-like animals, in which the 

 mantle envelops the whole body and bears numerous spicules but no 

 shell. There are two families, Neomeniidse and Chretodermidae. 



Of Neomeniidse, six genera are known. They have a longitudinal 

 pedal groove, an intestine without distinct digestive gland, two nephridia 

 with a common aperture, and hermaphrodite reproductive organs. The 

 ChDetodermidoe, represented by one genus Chcetoderma, are cylindrical 

 in form, without a pedal groove, with a radula bearing one tooth, with 

 a distinct digestive gland, and with two nephridia opening separately 

 into a posterior cavity, which also contains two gills. The sexes are 

 separate. 



There seem to be more than merely superficial resemb- 

 lances between these simple Molluscs and such worm 

 types as Turbellarians. It seems justifiable to speak of the 

 Amphineura as primitive Molluscs, but the Aplacophora 

 are perhaps rather degenerate than primitive. 



Class II. GASTEROPODA, e.g., Snail, Whelk, Limpet. 



GENERAL CHARACTERS. Gasteropods are more or less 

 asymmetrical Molluscs. The head region, which is well 

 developed, remains symmetrical, and so does the foot, which is 

 typically a flat creeping organ. But the visceral mass or 

 hump, with its mantle fold, is more or less twisted forwards 

 and to the right. Thus the pallial, anal, nephridial, and 

 genital apertures usually lie on the right side, more or less 

 anteriorly. A further asymmetry is shown by the twisting of 

 the morphologically right gill to the left side, while the original 

 left gill is usually lost. Similarly, one of the nephridia, pro- 

 bably that which is morphologically the left, tends to disappear, 

 and in most cases only one persists topographically on the left 

 side. The main torsion must be distinguished from the spiral 

 twisting which the visceral hump often exhibits, and from the 

 frequently associated spiral coiling of the univalve shell. 

 Moreover, a superficial secondary bilateral symmetry tends to 



