308 MANUAL OF ZOOLOGY. 



Fam. 20. Gastrochanida. — Shell equivalve, gaping, with thin eden- 

 tulous valves, sometimes cemented to a calcareous tube. Mantle- 

 margins thick in front, united, with a small pedal aperture. 

 Siphons very long, united. Foot finger-shaped. 111. Gen. Gas- 

 roc/icBna, Saxicava, Asper^illum. 



Fam. zi. Pholadida. — Shell gaping at both ends, without hinge ot 

 ligament, often with accessory valves. Animal club-shaped or 

 worm-like, with a short truncated foot. Mantle closed in front. 

 Siphons long, imited to near their extremities. 111. Gen. Pholas, 

 Xylo^haga, Teredo. 



CHAPTER XLVII. 

 GASTEROPODA. 



Division Encephala, or Cephalophora. — The remaining 

 three classes of the Mollusca proper all possess a distinctly 

 differentiated head, and are all provided with a peculiar masti- 

 catory apparatus, which is known as the " odontophore." For 

 the first of these reasons they are often grouped together under 

 the name EncepJiala ; and for the second reason they are 

 united by Huxley into a single great division, under the name 

 of Odontophora. Whichever name be adopted, the three classes 

 in question (viz., the Gasteropoda, Pteropoda, and Cephalopoda) 

 certainly show many points of affinity, and form a very natural 

 division of the Mollusca. The Pteropoda, as being the lowest 

 class, should properly be treated of first, but it will conduce to 

 a clearer understanding of their characters if the Gasteropoda 

 are considered first. 



Class II. Gasteropoda. — The members of this class are 

 characterised by being never included in a bivalve shell ; loco- 

 motion being effected by means of a broad, horizontally flat- 

 tened, ventral disc — the " foot ; " or by a vertically flattened, 

 ventral, fin-like organ. Flexure of intestine hjemal or neural. 



This class includes all those Molluscous animals which are 

 commonly known as " univalves," such as the land-snails, sea- 

 snails, whelks, limpets, &c. The shell, however, is sometimes 

 composed of several pieces (multivalve), and in many there is 

 either no shell at all, or nothing that would be generally recog- 

 nised as such. In none is there a bivalve shell. 



In their habits the Gasteropods show many differences, some 

 being sedentary, but the great majority being free and locomo- 

 tive. In these latter, locomotion may be effected by the sue- 



