198 STRUCTURE OF THE MOLLUSCA. 



lidia, having the head furnished with four tentacula, and 

 the arms on the hind-part of the mantle ; and the Diphyl- 

 lidia, having on each side of the head a pointed tentacuhun 

 and a slight tubercle, and the anus on the right side ; both 

 having the body oval, or more or less tubercular, belong to 

 this group. 



OEDER m-GASTEEOPODA TECTIBRAA^CHIATA. 



The Tectibranchiate Gasteropoda are also marine Mol- 

 lusca, but have the branchise unsymmetrical, and composed 

 of more or less divided laminse attached along the right side 

 or on the back, and more or less covered by the mantle, 

 which almost always contains in its substance a small shell. 

 To this order belong the Pleurobranchi, of an oval form, 

 with the branchiae fixed on the right side, in a groove be- 

 tween the mantle and the foot, a proboscidiform mouth, sur- 

 mounted by a small triangular veil, and two tentacula, four 

 stomachs, of which the second is sometimes armed with bony 

 pieces, and the anus behind the branchiae ; the Aph/sice, 

 which resemble slugs, but with the edges of the foot raised 

 up in the form of flexible crests, a long neck, four tentacula, 

 of which the upper are hollowed like the ears of a quadru- 

 ped, and placed above the eyes; the Dollabellce, which differ 

 from the Aplysise only in having the branchiae and mantle at 

 the posterior extremity of the body; and the Acera or Bulla:, 

 which resemble the Aplysiae in having the stomach armed 

 with a bony piece, but are distinguished by their tentacula, 

 which are short, broad, and disposed so as to form a large 

 fleshy shield over the eyes, some of them, the Acerce, being 

 naked; others, the Bullce, covered with a shell. 



