244 MOLLUSCA. 



the mouth of the shell is unbroken or " entire," not having 

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

 the mouth has this form are termed " holostomatous ; n and 

 for the most part they belong to Gasteropods which are phyto- 

 phagous, or live upon vegetable food. The possession, how- 

 ever, of a holostomatous 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. 206) 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 Siphonosto- 

 matous Univalves are mainly carnivorous 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 indicates that the animal possessed tubular 

 inflections of the mantle, or " respiratory siphons," by which 

 the water is conveyed to and from the gills. 



Divisions of the Gasteropoda. The following table shows 

 the chief divisions of the Gasteropoda : 



TABLE OF THE GASTEROPODA. 



SECTION A. BRANCHIFERA. Respiration aquatic, by the walls of the 

 mantle-cavity or by gills. 



ORDER I. PROSOBRANCHIATA. The branchiae situated (prosori) 

 in advance of the heart. 



Division a. Siphonostomata. Margin of the shell-aperture 

 notched or produced into a canal. This division comprises the 

 families of the Strombidcc (Wing-shells), Muricidcc, Buccinidce 

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

 (Cowries). 



Division b. Holostomatd. Margin of the shell-aperture " en- 

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

 includes the families of the Naticidce, Pyramidellid<z, Cerithiada, 

 Melaniadce, Turritellidcz, Littorinidce (Periwinkles), Paludinidce 

 (River-snails), Neritida, Turbinid<z (Top-shells), Haliotida % Fis- 

 stirellidtz (Keyhole-limpets), Calyptrceidtz (Bonnet-limpets), Patel- 

 lidce (Limpets), Dentalidce (Tooth-shells), and Chitonidce. 

 ORDER II. OPISTHOBRANCHIATA. Branchise placed towards the 

 rear {opisthen} of the body. 



Division a. Tedibranchiata. Brdnchia covered by the shell or 

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

 families of the Tornatellidce, Bullidce (Bubble-shells), Aplysiada 

 (Sea-hares), PUurobranchiadae^ and Phyllidiada. 



Division b. Nudibranchiata. Animal destitute of a shell in the 

 adult condition. Branchiae external, on the back or sides of the 

 body. This division includes the various naked Gasteropods 

 commonly known as Sea-lemons and Sea-slugs. 

 ORDER III. NUCLEOBRANCHIATA, or HETEROPODA. Shell pre- 

 sent or absent. Animal free-swimming and oceanic, with a fin-like 



