DENTAL1UM. OPITIIOBRANCHIATA. 385 



sembling the tusk of an elephant, whence the name of Tooth- 

 shells is ordinarily applied to them. The animal is of a cylin- 

 drical form, inclosed in a sac-like mantle, from an aperture at 

 the anterior extremity of which the foot can be protruded. 

 Above the base of the foot, near the middle of the body, the head 

 is situated ; and on each side of this is a tuft of numerous cirrus- 

 like organs, which have been commonly regarded as branchiae, al- 

 though from recent researches it would appear that they are 

 rather to be considered as tentacular organs. The shell has an 

 aperture at each extremity ; its surface is sometimes smooth, and 

 sometimes longitudinally striated. The Dentaliidae occur in 

 most seas, inhabiting a sandy or muddy bottom, in which they 

 usually bury themselves by means of the foot. They are carni- 

 vorous, feeding upon minute marine animals. 



ORDER III. OPISTHOBRANCHIATA. 



1002. In the second Order of branchiferous Gasteropods, the 

 gills are not inclosed in a regular chamber, but either exposed 

 on the back of the animal, or more or less concealed by folds of 

 the mantle. They are usually of a tufted or arborescent form, 

 and placed on the back or sides of the body, behind the heart. 

 The Mollusca of this Order, which might be designated Sea-slugs, 

 are usually destitute of a shell ; and in those genera which pos- 

 sess a shell, this is almost always internal, and generally rudi- 

 mentary. These animals are all marine, and for the most part 

 carnivorous in their habits. The Order may be divided into two 

 principal sections, in accordance with differences in the position 

 of the gills. In the NUDIBRANCHIATA, these organs are freely 

 exposed on the back or sides of the body ; and the animals possess 

 no shell either internal or external. In the TECTIBRANCHIATA, 

 on the contrary, the animal is frequently provided with a shell, 

 and the branchiae are concealed either by the mantle or by the 

 shell, when the latter is present. 



