372 



MOLLUSC A. 



varied. A twisted gut runs through the body, surrounded by a diffuse 

 digestive gland. There is a radula in the mouth. The heart is median 

 and posterior, and consists of a ventricle and two to eight auricles. There 

 are two symmetrical nephridia opening posteriorly, and consisting of 

 much-branched tubes. 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. Neomenia, 

 Proneomenia, and Chcetoderma. 



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 ChsetodermidEe. 



Of Neomeniida;, 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 

 Chsetodermidse, represented by one genus Chtetoderma, 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. 



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 

 be acquired by free-swimming forms, e.g. Heteropods. The 



