SWAMMERDAM 195 



shell, he explains, is not the house of the snail, but part 

 of its body ; it is formed of " true bone," and has 

 muscles inserted into it. The tentacles are carefully 

 and admiringly described. He shows how they are 

 withdrawn by special retractor muscles, and protruded 

 by a kind of peristaltic action of their annular muscles. 

 The eye on the tip of the tentacle receives close atten- 

 tion ; Swammerdam finds in it all the layers of the 

 vertebrate eye, and even believes that it possesses an 

 iris, though he admits that he has not seen it. The 

 eyes of the snail are, he tells us, ineffective for the 

 perception of near objects. He notices in passing the 

 eye of the mole, which also is of little or no use ; to 

 this keen-sighted anatomist it was so plain that he could 

 dissect it without a microscope. He figures the jaw 

 of the apple-snail and its odontophore, but he does 

 not seem to have found the lingual ribbon,^ one of those 

 exquisite contrivances in which he was accustomed to 

 take delight. It was not only Swammerdam's eyes 

 which were quick to perceive ; he speaks of hearing 

 the sound which the snail makes in feeding. Then 

 he goes on to the organs of respiration and circulation. 

 His knowledge of the circulation is not quite complete ; 

 for example, he takes the pulmonary vein to be the vena 

 cava. He tells how from this vessel the heart and 

 arteries can be inflated, or filled with a coloured liquid. 

 He knows that the blood of the snail is coagulable, and 

 that it turns milky when mixed with water. This leads 

 him to expose the mistake of speculative writers in 

 saying that small animals '*of this kind" (he means 

 invertebrate animals) have no blood ; they really have 

 blood, but, except in the earthworm, it has no crimson 

 colour. He mentions and figures the renal organ, which 



^He desoribos elsewhere the lingual ribbons of Paludina and Sepia. 



