MOLLUSCA : GASTEROPODA. 



2 9 I 



branchial^ the Opisthobranchiata, and the Nudeobranchiata or 

 Heteropoda. 



ORDER I. PROSOBRANCHIATA. The members of this order 

 are defined as follows : " Abdomen well developed, and pro- 

 tected by a shell, into which the whole animal can usually 

 retire. Mantle forming a vaulted chamber over the back of 

 the head, in which are placed the excretory orifices, and in 

 which the branchiae are almost always lodged. Bronchia 

 pectinated or plume-like, situated (prosori) in advance of the 

 heart. Sexes distinct." M.-Edwards. (See Woodward's 

 ' Manual.') 



The order Prosobranchiata includes all the most character- 

 istic members of the Branchiate Gasteropods, and is divisible 

 into two sections, termed respectively Siphonostomata and 

 Holostomata, according as the aperture of the shell is notched 

 or produced into a canal, or is simply rounded and " entire." 



The Siphonostomata, of which the common Whelk (Bucci- 

 num undatum} may be taken as an example, are all marine, 

 and are mostly carnivorous in their habits. The following 

 families are comprised in this section : Strombidce (Wing- 

 shells), Muritidce, Buccinida (Whelks), Conida (Cones), Volu- 

 tida, and Cypraida (Cowries). 



The Holostomata, of which the Common Periwinkle (Litto- 



Fig. 103. Gasteropoda, a Holostomatous shell ( Turri fella communis] \ 

 b Siphonostomatous shell (Buccinum undatuni). 



rina littorea) is a good example, are either spiral, or limpet- 

 shaped, in some few instances tubular, or multivalve ; the aper- 

 ture of the shell being in most cases entire. They are mostly 



