286 MANUAL OF ZOOLOGY. 



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

 chcena, Saxicava, Aspergillum. 



Fam. 21. Pholadidce. Shell gaping at both ends, without hinge or 

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

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

 Siphons long, united to near their extremities. 111. Gen. Pholas t 

 Xylophaga,) 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 Encephala ; and for the second reason they are 

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

 of Odonlophora. 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 haemal 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 

 recognised 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 

 locomotive. In these latter, locomotion may be effected by 

 the successive contractions and expansions of a muscular foot ; 

 but some possess the power of swimming freely by means of 

 a modified fin-like foot. 



