478 



PHYLUM MOLLUSCA 



Class II. SOLF.NOGASTRES 



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

 mantle envelops the whole body and bears numerous spicules, but no 

 shell. It is somewhat doubtful if they are Molluscs at all. There are 

 two families — Neomeiiiidae and Chstodermidae. 



Of Neomeniidfe, six genera are known, e.g. Neomenia and Pro- 



neomenia. They have a longitudinal 

 pedal groove, an intestine without 

 distinct digestive gland, two neph- 

 ridia with a common aperture^ and 

 hermaphrodite reproductive organs. 

 The Chaetodermida,', represented by 

 one genus Chcetoderma, are cylin- 

 drical 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 III. SCAPHOPODA 



Very different in many respects 

 from Gasteropoda are the Scapho- 

 poda, of which Dentalium (Ele- 

 phant's tooth-shell) is the com- 

 monest genus. They are apparently 

 related to the Zygobranchiate Gas- 

 teropods, and also to the simplest 

 Bivalves. They burrow in the sand 

 at considerable depth off the coasts 

 of many countries. The mantle has 

 originally two folds, which fuse 

 ventrally, and the shell becomes 

 cylindrical, like an elephant's tusk. 

 It is open at both ends. The 

 larger opening (directed downwards 

 in the sand) is anterior, the concave 

 side of the shell is dorsal. The 

 mouth opens at the end of a short 

 buccal tube, at the base of which 

 is a circle of ciliated tentacles. The 

 foot is long, with three small terminal lobes. It is used in slow creeping, 

 and is protruded at the anterior opening. There are cerebral and 

 pleural ganglia near one another in the head, pedal ganglia in the foot, 

 and a long untwisted visceral loop with olfactory ganglia near the 

 posterior anus. Sense organs are represented by otocysts beside the 

 pedal ganglia. There is an odontophore with a simple radula. The 

 food consists of minute animals. There is a much reduced heart, and 



Fig. 273. — Proneomenia. Ner- 

 vous system. — From Hubrecht. 



e.g.. Cerebral ganglia ; s/g., sublingual ; 

 a.p.g., anterior pedal ; p.p-g., pos- 

 terior pedal ; p.v.g., posterior vis- 

 cerals ; si., sublingual connectives ; 

 cpc, cerebro-pedal connective ; pe., 

 longitudinal pedal nerves ; la., longi- 

 tudinal lateral nerves. 



