STRUCTURE OF THE MOLLUSCA. 197 



which the water arrives at a slit in the right side of the 

 mantle, and penetrates into the branchial cavity. Round the 

 foot is a double membrane cut into laminae and furnished 

 with long filaments. Fissurella has a shell like that of a 

 limpet, but with a hole in the top, which afifords passage to 

 the water necessary for respiration, as well as to the faeces. 

 The EmarginulcB present in front a notch or slit, for the 

 same purpose. 



OEDER V -aASTEEOPODA CYCLOBEANCHIATA. 



The Cyclobranchiata are very nearly allied to the Scuti- 

 branchiata ; but are distinguished by having their branchise 

 fixed beneath the edges of the mantle. The PatelUna are 

 furnished with a conical shell, which covers the whole body. 

 They have two pointed tentacula, bearing the eyes at their 

 base ; a short proboscis, and a circular series of respiratory 

 lamellae between the mantle and the foot. The Chitonina 

 differ from all other Mollusca in having an oblong depressed 

 shell, composed of eight plates, kept together by ligaments, 

 and cased in a thick margin. A membranous veil over the 

 mouth answers for tentacula; and the branchiae, composed 

 of triangular lamellae, are placed under the margin of the 

 mantle. 



OEDEE VI.-GASTEEOPODA MEEEOBEANCHIATA. 



The Inferobranchiata, which are very few, are naked 

 Mollusca, having their branchiae composed of a long series 

 of laminae, and placed on the two sides of the body, between 

 the foot and the edge of the mantle. They differ from the 

 Cyclobranchiata not only in being destitute of shell, but 

 also in some points of their internal structure. The Phyl- 

 r3 



