258 MANUAL OP ZOOLOGY. 



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

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

 Xylophaga, Teredo. 



CHAPTER XLVII. 



GASTEROPODA. 



DIVISION ENCEPHALA OR CEPHALOPHOEA. The remaining three 

 classes of the Mollusca proper all possess a distinctly differen- 

 tiated head, and are all provided with a peculiar masticatory 

 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 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 hasmal or neural. 



This class includes all those Molluscous animals which are 

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

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

 composed of several pieces (multi valve), 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. 



In most of the Gasteropoda the body is unsymmetrical, and is 

 coiled up spirally, ' the respiratory organs of the left side being 

 usually atrophied.' (Woodward.) The body is enclosed in a 

 ' mantle,' which is not divided into two lobes as in the Lamel- 

 lilranchiata, but is continuous round the body. Locomotion 



