GASTEROPODA. 



any notch or indentation of its margin. The shells in which 

 the mouth has this form are termed " holostomatons ; " and 

 for the most part they belong to Gasteropods which are 

 phytophagous, or live upon vegetable food. The possession, 

 however, of a holostomatons shell in reality simply proves 

 that the animal had no respiratory " siphons," or tubes 

 formed by the folding of the mantle. In a second group the 

 aperture of the shell (fig. 385) is notched in front; and the 

 shell is said to be " siphonostomatous." There may be a 

 posterior notch as well as the anterior one, and one or both 

 of these notches may be produced into longer or shorter 

 canals. The Siphonostomatous Univalves are mainly car- 

 nivorous in their habits ; but the notched mouth does not 

 necessarily indicate the nature of the food. The possession 

 of a siphonostomatous shell, on the contrary, merely in- 

 dicates that the animal possessed tubular inflections of the 

 mantle, or " respiratory siphons," by which the water is con- 

 veyed to and from the gills. 



Divisions of the Gasfcro2Joda. — The following table shows 

 the chief divisions of the Gasteropoda : — 



Table of the Gasteropoda. 



Section A. Bbanchifera. — Respiration aquatic, by tlie walls of the 

 mantle-cavity or by gills. 



Order I. Prosobranchiata. — The branchias situated (jproson) 

 in advance of the heart. 



Division a. Siphonostomata. — Margin of ike ^ sliell - aperture 

 notched or produced into a canal. This division comprises the 

 families of the Strombida; (Wing-shells), Muricidce, Buccinidce 

 (Whelks), Conidce (Cones), Volutidce (Volutes), and Cyprceidce 

 (Cowries). 



Division b. Holostomata. — Margin of the shell- aperture ^^ en- 

 tire," rarely notched or produced into a canal. This division 

 includes the families of the Naticidce, PyramidellidxB, Cerithiadai, 

 Melcmiada', Turritellidai, Littorinida; (Periwinkles), Paludinidce 

 (River-snails), Neritidce, Turbinidce (Top-shells), Haliotidw, Fis- 

 surellidce (Keyhole-limpets), Cahjptraidai (Bonnet-limpets), Patel- 

 Udce (Limpets), Dentcdidce (Tooth-shells), and Chitonida;. 

 Order II. Opisthobranchiata. — Branchiae placed towards the 

 rear (opisthen) of the body. 



Division a. Tectibranchiata. — Branchice covered by the shell or 

 mantle. A shell in most. Sexes united. The division includes 



