234 LIFE BY THE SEASHORE. 



operculum (o in Fig. 70), so placed as to block the mouth of 

 the shell when once the animal has withdrawn into it. If 

 the chief function of the shell be for protection, then the 

 shell of the Azygobranchs is more efficient than the shell of 

 the Zygobranchs. 



While you have been making these observations, some of 

 your specimens will have recovered from their alarm, and 

 have begun to crawl about the basin. In such expanded 

 specimens notice as before the creeping foot (/ in Fig. 70), 

 not unlike that of the snail, and also the relation of the 

 operculum to it. A comparison of forms with notched and 

 unnotched shell will show further that in the former a long 

 tube or siphon (see Fig. 70, s) can be protruded along the 

 canal or notch. This siphon is a specialised portion of the 

 mantle, and conveys water to the mantle-chamber in which 

 lies the gill. You will remember that in the tortoise-shell 

 limpet the gill itself is protruded as the animal walks ; but 

 gills are delicate, easily injured structures, full of blood, 

 and not to be exposed without some risk, therefore we 

 find that with the specialisation which gives the Azygo- 

 branchs their more complex shells, there is usually more 

 efficient protection for the gill, which is now usually hidden 

 permanently within the mantle-chamber. Lest, however, 

 in this position the gill should not be sufficiently exposed to 

 the purifying action of the water, there is in many cases a 

 long siphon which conveys a current of water to the mantle- 

 chamber. We have already noticed a similar condition of 

 affairs in the Crustacea, where in the higher forms the 

 position of the gills in a gill -chamber necessitates very 

 elaborate arrangements for renewing the water. Curiously 

 enough it is found that almost all the siphonate Azygo- 

 branchs are carnivorous, while those without siphons are 

 vegetarian. The former are more specialised than the latter. 



Notice, also, that the Azygobranchs have a much better 

 developed head region than the limpet. It is often prolonged 

 into a proboscis, which may, as in the whelk, be capable of 

 being protruded and retracted. The tentacles are often very 

 long, and in some cases, as in the tops, there are numerous 

 tactile processes in addition to the tentacles proper. 



While the limpets lay their eggs singly in the water, the 

 Azygobranchs lay them in clusters or capsules which arc 



