GASTEROPODA. 



103 



like litiopa and rissoa parva, which, anchor themselves 

 to sea-weeds (Gray), and cerithldea (fi^. 68), which 

 fi-equeutly leaves its proper element, and is found 

 hanging in the air (Adams). A West India land- 

 snail {c.ydostoma suspensitm) also suspends itseK (Guild- 

 ing). The origin of these threads has not been ex- 

 plained ; but some of the limaces lower themselves to 

 the gi'ouud by a thread which is not secreted by any 

 particular gland, but derived from the exudation over 

 the general surface of the body (Lister; D'Orbigny). 



The division of this extensive class into orders and 

 families, has engaged the attention of many natm-alists, 

 and a variety of methods have been proposed. Cu- 

 vier's classification was the first that possessed much 

 merit, and several of his orders have since been united 

 w^th advantage. 



Fig. 



ORDER I. Prusobranchiata. 

 Abdometi well developed, and protected 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. BranchioB pectinated, or plume-like, 

 situated {proson) in advance of the hcai't. Sexes distinct. (M. Edwards.) 



SECTION A. SiPHONOSToMATA. Carnivorous Gastcropods. 



Shell spiral, usually imperforate ; apertm-e notched or produced into a 

 canal in front. Operculum horny, lamellar. 



Animal provided with a retractile proboscis ; eye-pedicels connate with the 

 tentacles ; margin of the mantle prolonged into a siphon, by which water is 

 conveyed into the branchial chamber ; gills 1 or 2, comb-like, placed obliquely 

 over the back. Species all marine. 



