MOLLUSCA: GASTEROPODA. 405 



CHAPTER XLV. 



GASTEROPODA. 



DIVISION ENCEPHALA, or CEPHALOPHORA. The remaining 

 three classes of the Mollusca proper all possess a distinctly 

 differentiated head, and all are 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 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 

 never included in a bivalve shell ; locomotion is effected by means of 

 a broad, horizontally flattened, ventral disc the "foot; " or by a 

 vertically flattened, ventral, fin-like organ. Flexure of intestine 

 hcemal or neural. 



This class includes all those Molluscous animals which have 

 a shell of a single piece, and are commonly known as " uni- 

 valves," 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. The Gasteropods may be regarded as 

 the most typical of the Mollusca, though not the most highly 

 organised. All of them have a body composed of three prin- 

 cipal portions a head, foot, and visceral sac the last of these 

 being enclosed in the integumentary expansion known as the 

 " mantle." In all, except the few sedentary forms, the " foot " 

 is the organ of locomotion. 



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- 

 libranchiata, but is continuous round the body. Locomotion 

 is effected by means of the "foot," which is usually a broad 

 muscular disc, developed upon the ventral surface of the body, 



