GASTEROPODA. 179 



ORDER III. OPISTHO-BRANCHIATA. 



Shell rudimentary or wanting. Branchiae arborescent or fasciculated, 

 not contained in a special cavity, but more or less completely exposed on the 

 back and sides, towards the rear (opisthen) of the body. Sexes united. 

 (M. Edwards}. 



The molluscs of this order may be termed sea-slugs, since the shell, when 

 it exists, is usually small and thin, and wholly or partially concealed by the 

 animal. When alarmed or removed from their native element, they retract 

 their gills and tentacles, and present such a questionable shape that the in- 

 experienced naturalist will be likely enough to return them, with the refuse 

 of the dredge, into the sea. Their internal structure presents many points 

 of interest ; in some the gizzard is armed with horny spines, or large shelly 

 plates ; in others the stomach is extremely complicated, its ramifications and 

 those of the liver being prolonged into the branches of the respiratory organ. 

 The tongue is always armed, but the number and arrangement of the lingual 

 teeth is exceedingly variable, even in the same family ; usually the dental 

 membrane is broad and short, with many similar teeth in each row. The 

 alimentary canal terminates more in the rear of the body than in the other 

 univalve shell-fish.* The gills are behind the heart, and the auricle behind 

 the ventricle; conditions which characterize the embryonic state of the 

 mollusca generally. 



Comparatively little is known of the geographical distribution of these 

 animals ; they have been found wherever the requisite search has been made, 

 and are probably much more numerous than at present estimated. The 

 shell-bearing genera flourished in the period when the secondary strata were 

 deposited. The living species are chiefly animal-feeders, preying on other 

 shell-fish and on zoophytes. 



SECTION A. TECTI-BRANCHIATA.f 



Animal usually provided with a shell, both in the larval and adult state ; 

 branchiae covered by the shell or mantle ; sexes united. 



FAMILY I. TORNATELLID^. 



Shell external, solid, spiral or convoluted, sub-cylindrical ; aperture long 

 and narrow ; columella plaited ; sometimes operculated. 



Animal with a flattened, disk-like head, and broad obtuse tentacles ; foot 

 ample, furnished with lateral and operculigerous lobes. 



* In the cuttle-fishes and pteropods it is hent upon itself ventrally, in the sea- 

 snails dorsally, terminating in front, near its origin; the vascular system partakes of 

 this flexure, and the gills are in advance of the heart. (Huxley.} 



t Mono-pluuro-branchiata. SI. Pomato-branchia, (from poma, a lid). Wiegm. 

 The orderTecti-branchiata of Cuvier included only the family Bullidce ; it is here 

 made to comprise the Infero-branches also; no object being gained by the multipli- 

 cation of descriptive epithets. 



