* 
GASTEROPODA. 209 
Some of the gasteropoda can suspend themselves by glutinous 
threads, like litiopa and Rissoa parva, which anchor themselves 
to sea-weeds (Gray), and cerithidae (Fig. 74), 
which frequently leaves its proper element, / 
and is found hanging in the air. (Adams.) A /¥ 
West India land-snail (cyclostoma suspensuwm) * 
also suspends itself. (Guilding.) The origin 
of these threads has not been explained ; 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. Cuyier’s classification was Fig. 75. 
the first that possessed much merit, and several of his orders 
haye since been united with advantage. 
System of Cuvier. System now adopted. 
“lass. GASTEROPODA. 
Order 1. Pectinibranchiata 
a ERE Ord. Prosobranchiata, M. Edw. 
4, Tubulibranchiata 
5. Pulmonata Ord. Pulmonifera. 
6. Tectibranchiata 
7. Inferobranchiata i Ord. Opisthobranchiata, M. Edw. 
8. Nudibranchiata. 
Class. HETEROPODA. Ord. Nucleobranchiata, Bl. 
ORDER I. PROSOBRANCHIATA. 
Abdomen well developed and protected by a shell, into which 
the whole animal can usually retire. Mantle forming a vaulted 
chamber oyer the back of the head, in which are placed the 
excretory orifices, and in which the branchie are almost always 
lodged. Branchie pectinated, or plume-like, situated (proson) 
in adyance of the heart. Sewes distinct. (M. Edwards.) 
Szotion A. SIPHONOSTOMATA. Carnivorous Gasteropods. 
Shell spiral, usually imperforate; aperture notched or produced 
into a canal in front. Operculum horny, lamellar. 
