2l6 



SIZE OF RADULA PRESENCE OR ABSENCE 



Strutkiolaria, 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 l)readth ; in most other 

 groups the breadth is inconsider- 

 able, as compared to the length. 



The radula is wanting in two 

 families of Prosobranchiata, the 

 Eulimidae and Pyramidellidae, 

 ^vhich are consequently grouped 

 together ns the section Gymno- 

 glossa. It is probable that in 



these cases the radula has aborted 

 Fig. 112. — Example of a front portion ,i it j.i ■ ^ ^ 



of a r^,\u\^ {Cantharvs ringens through dlSUSC, the animals liav- 

 Eeeve, Panama), much worn by use. ing taken to a food which doeS 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 compensates for 

 the loss of radula (see pp. 70-77). In Harpa there is no radula in 

 the adult, though it is present in the young form. ^No explana- 

 tion of this fact has yet l)een given. It is also absent in the 

 Coralliophilidae, a family closely akin to FmyMra, 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 t^ynapta, or in Neomenia, a genus of very low 

 organisation, or in the Tethyidae, or sea-hares, or in one or two 

 other genera of Nudiljranchiata. 



The number of teeth in the radula varies greatly. "When the 

 teeth are very large, they are usually few in numljer, when small, 

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

 teeth are comparatively few and powerful, ^vhile in the phyto- 

 phagous 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. Genera which are 



