SEA-SHELLS ON THE SEASHORE 149 



have large spiral shells, into which the whole of the soft 

 animal can be deeply withdrawn, which is not the case 

 with the limpet. You may find on the shore at Torquay 

 a sea-snail (Natica), in which the animal is quite invisible, 

 drawn far up into the shell. Place this in sea-water and 

 watch it. Soft semi-transparent lobes begin to issue 

 from the mouth of the shell, part of the soft distensible 

 foot appears swelling out and growing bigger and 

 bigger, and soft folds spread out from the mouth 

 of the shell, and gently creep over it, and completely 

 envelop it ; the foot begins to grip the bottom of the 

 vessel, and the animal "crawls." At last, swelling 

 out from the other folds of soft but tense " molluscan " 

 substance, the head and its tentacles emerge. Touch 

 the animal and it shrinks rapidly, disappearing into 

 the shell. 



It used to be thought (about twenty-five years ago) 

 that the molluscs expand their bodies in this manner by 

 taking water, through definite apertures provided with 

 valves, into their blood, and that, having thus swelled 

 themselves out, they could shrink and reduce themselves 

 by pouring out again the in-taken water. The behaviour 

 of some other marine animals, namely the sea-anemones, 

 which really do act in this way, made this explanation 

 of the swelling and shrinking of molluscs seem probable. 

 It was also known that the star-fishes and sea-urchins 

 actually do take in the sea-water into a system of vessels 

 connected with their wonderful sucker-bearing tentacles. 

 But it turned out on close examination that the molluscs 

 do not take in or shed out water in this way. A hole, 

 which was thought to let in water into the blood of sea- 

 snails, was shown to be only the opening of a great slime- 

 gland. In the case of some bivalves which have red- 

 blood corpuscles, I showed that the blood is never made 



