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 ; " 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 (proson) 

 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 Strombidtz (Wing-shells), Muricidte, Buccinida 

 (Whelks), Conida (Cones), Volutidie (Volutes), and Cyprcrida 

 (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 Naticida, Pyramidellida, Cerithiad<e, 

 Afelaniadtf, Turritellida:, Littorinida: (Periwinkles), Paludinidte 

 (River-snails), Neritida, Turbinidtz (Top-shells), Haliotidte, Fis- 

 surellidtz (Keyhole-limpets), Calyptrtrida (Bonnet-limpets), Palel- 

 lida (Limpets), Dentalida (Tooth-shells), and Chitonidcc. 

 ORDER II. OPISTHOBRANCHIATA. Branchiae placed towards the 

 rear (opistheit) of the body. 



Division a. Tectibranchiata. Brdnchitt covered by the shell or 

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

 families of the Tornatellidif, Bullida; (Bubble-shells), Aplysiada 

 (Sea-hares), Pleurobranehia<Le> and Phyllidiada;. 



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

 adult condition. Bronchia 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 



