2l6 



SIZE OF RADULA — PRESENCE OR ABSENCE 



Struthiolaria, and the Cephalopoda it is small in proportion to 



the size of the animal. In the Pul- 

 monata generally it is very broad, 

 the length not exceeding, as a rule, 

 thrice the breadth; in most other 

 groups the breadth is inconsider- 

 able, as compared to the length. 



The Radula is wanting in two 



families of Prosobranchiata, the 



^^\ \ I iy\3\ Eulimidae and Pyramidellidae, 



\ ^ Y^ — ^ ^% which are consequently grouped 



^-yO L'-J €?^^ together as the section Gymno- 



/^^ ^^1 glossa. It is probable that in 



' these cases the radula has aborted 



^'';f a7a?urrJ^;?.irr;:« through disuse, the ani^als hay- 



Reeve, Panama), much worn by ing taken tO a food which doCS not 



^^' ^ ' ' require trituration. Thus several 



genera contained in both these families are known to live para- 

 sitically upon various animals — Holothurians, Echinoderms, etc. 

 — nourishing themselves on the juices of their host. In some 

 cases, the development of a special suctorial proboscis compen- 

 sates for the loss of radula (see pp. 76-77). In Harpa there is 

 no radula in the adult, though it is present in the young form. 

 No explanation of this fact has yet been given. It is also absent 

 in the Coralliophilidae, a family closely akin to Purpura^ but 

 invariably parasitic on corals, and probably nourished by their 

 exudations. There is no radula in Entoconcha^ an obscure form 

 parasitic on the blood-vessels of Synapta^ or in Neomenia, a 

 genus of very low organisation, or in the Tethyidae, or sea- 

 hares, or in one or two other genera of Nudibranchiata. 



The number of teeth in the radula varies greatly. When 

 the teeth are very large, they are usually few in number, when 

 small, they are very numerous. In the carnivorous forms, as a 

 ' rule, the teeth are comparatively few and powerful, while in 

 the phytophagous genera they are many and small. Large 

 hooked and sickle-shaped teeth, sometimes furnished with barbs 

 like an arrow-head, and poison-glands, are characteristic of 

 genera which feed on flesh ; vegetable feeders, on the contrary, 

 have the teeth rounded, and blunter at the apex, or, if long 

 and narrow, so slender as to be of comparatively little effect. 



