308 



IN VERTEBRA TE MORPROLOG T. 



tooth on either side peculiarly long, forming the toxiglossate 

 arrangement with a formula 1, 0, 1 ; and finally certain forms, 

 such as lanthina, Scalaria (Fig. 137, U), Solarium, etc., have a 

 ptenoglossate arrangement in which the median is wanting 

 but in which there are a large number of admedians, x, 0, x. 



The suborder is relatively very rich in species, and conse- 

 quently considerable variety of form is found. The majority 

 are marine, but a few are fresh-water or even terrestrial in 

 habitat. In these latter adaptations to their mode of life are 

 found in modifications of the respiratory processes. In Am- 



FIG. 137. A, DENTITION OF Natica ; B, OF Nassa ; G, OP Pleurotoma ; D, 



OP Scalaria (from BRONN). 



pullaria the single branchia persists, but in addition a com- 

 paratively capacious " lung-cavity " is formed by a fold of the 

 mantle, its walls being richly supplied with blood-vessels and 

 its cavity being in communication with the exterior, so that 

 air can be taken into and expelled from it. The species of 

 this genus live partly in fresh water and partly are terrestrial, 

 but in other forms, such as Cyclostoma, which are purely ter- 

 restrial, the branchia has entirely aborted, respiration being 

 aerial and performed by the highly vascular wall of the 

 mantle-cavity. 



The majority of the marine Monotocardia have a creep- 

 ing habit, but a number are pelagic and form a group pre- 

 senting many adaptive peculiarities which obtained for it the 

 dignity of an order in older classifications. The members of 

 this group, HETEROPODA, are more or less transparent animals, 

 some of which, Avith this exception, present few differences 



