GASTEROPODA. 



103 



like litiopa and rissoa parva, which anchor themselves 

 to sea-weeds (Gray), and cerithidea (fig. 68), which 

 frequently leaves its proper element, and is found 

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

 snail (cyclostoma suspensum) also suspends itself (Guild- 

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

 plained ; but some of the limaces lower themselves to 

 the ground 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 naturalists, 

 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 

 with advantage. 



Fig. 68. 



System of Cuvier. 

 Class. GASTEROPODA. 



Order 1. Pectinibranchiata 



2. Scutibranchiata 



3. Cyclobranchiata 



4. Tubulibranchiata 



5. Pulmonata 



6. Tectibranchiata 



7. Inferobranchiata 



8. Nudibranchiata 

 Class. HETEROPODA. 



System now adopted. 



Ord. Prosobranchiata, M. Edw. 



Ord. Pulmonifera. 



Ord. Opisthobranchiata, M. Edw. 



Ord. Nucleobranchiata. Bl. 



ORDER I. PROSOBRANCHTATA. 



Abdomen 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. Branchiae pectinated, or plume-like, 

 situated (proson) in advance of the heart. Sexes distinct. (M. Edwards.) 



SECTION A. SIPHONOSTOMATA. Carnivorous Gasteropoda. 



Shell spiral, usually imperforate ; aperture 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 

 o\er the back. Species all marine. 



