vii MOLLUSC A SYSTEMATIC REVIEW 3 



sole for creeping or for attachment. The sexual ducts and the nephridia are paired. 

 The sexes are separate. The heart is provided with two auricles. Radula (3 + 1), 

 (2 + 1), ( 1 + 1 + 1), (1 + 2), (1 + 3). Chiton (Fig. 1 ), Chitonellus. 



OKDER 2. Aplacophora sive Solenogastres. 1 



The body is almost cylindrical, and generally vermiform. There is no shell. 

 The much thickened cuticle contains calcareous spicules. The foot is rudimentary, 

 a mere ridge being left, and the mantle cavity is reduced to a groove at the sides of 

 this ridge, and a cavity (cloaca) at the posterior part of the body, into which the 

 intestinal canal and nephridia open, and in which are found, when present, the 

 rudimentary gills. The nephridia serve as ducts for the genital products. 



Family 1. Neomeniidae. 

 The foot is a longitudinal ridge, which rises from the base of a medio-ventral 



FIG. 2. Proneomenia Sluiteri, two-thirds natural size. A, From the right side ; B, from 

 beneath ; o, mouth ; cl, cloaca. 



longitudinal furrow. This family is hermaphrodite. Proneomenia (Fig. 2), Neo- 

 menia, Lepidomenia, Dondersia. 



Family 2. Chaetoderniidse. 



The foot and the pedal furrow are quite degenerated. The sexes are separate. 

 Chcetoderma. 



CLASS II. Gastropoda (Cephalophora). Snails. 



The body is asymmetrical. The head, which carries tentacles and eyes, is 

 generally distinct from the body. The foot is well developed usually with a flat sole 

 for creeping. The large protruding visceral dome may be flattened down secondarily 

 in all the groups. It is covered by a shell, consisting of a single piece, into which the 

 animal can withdraw. In all divisions, however, though rarely among the Proso- 



1 Simroth, in the new edition of Bronn's Klassen und Ordnungen des Thierreiches, 

 vol. iii., 1893, divides the Solenogastres as follows : 



Sub-Order. Fam. 



Chsetoderniatina . . Chaetodermatidae. 



( Neomeniidae. 

 ) Proneomeniidae. 

 Neomenima . j Dondersiidge . 



( Paramenhdae. 



