STREPTONEURA 



491 



radula is composed of very strong teeth and there are a small number 

 of marginals (docoglossate type). This type of radula is suited for the 

 feeding habits of the limpet, which scrapes the crust of minute algae 

 off the surface of rocks. Limpets have a remarkable "homing" sense, 

 returning after excursions for food to the same spot, which may be 

 marked by a depression in the rock. 



Fig. 363. To illustrate origin of euthyneury in the Pulmonata, A, B, and the 

 Opisthobranchiata, C, D. After Naef. A, Chilina. The left parietal ganglion 

 (l.pa.g.) has moved forward owing to the shortening of its pleural connective. 

 B, A pulmonate belonging to the Basommatophora. The corresponding con- 

 nective on the other side has shortened also, the visceral loop has become 

 untwisted and the nerve ganglia are concentrating. C, Actaeon, with short 

 spire and broad shell mouth, ctenidium and anus pointing to the right. 

 D, Bulla, showing slightly greater detorsion without spire, the shell mouth 

 opening to the right and anus pointing posteriorly: left parietal ganglion 

 drawn over right connective so that visceral loop is untwisted, an. anus; 

 au. auricle; ct. ctenidium; l.pa.g. left, r.pa.g. right parietal ganglion; ma.c. 

 mantle cavity; v. ventricle. 



Suborder MONOTOCARDIA 



Buccinum, the whelk (Fig. 356 D), lives between low-water mark and 

 100 fathoms. It is active and carnivorous, feeding on living and dead 

 animals, which it grasps by means of its foot. It has a remarkable 



