MARINE MOLLUSCS 



229 



runs from the body internally to the tip ; and, by the contraction 

 of this muscle the tentacle is involuted just in the same way as the 

 finger of a glove could be by pulling a string attached to the tip 

 inside. In addition to these tentacles, and the eyes and mouth 

 previously mentioned, the head is furnished with ear-sacs, which 

 are little cavities, filled with fluid containing solid particles, with 

 nerve filaments distributed in the walls. 



On the floor of the mouth there is a ribbon, supported on a base 

 of gristle, and covered with numerous minute teeth arranged 

 regularly in rows. The gristle is moved backwards and forwards 

 by means of muscles in such a manner that this ' lingual ribbon ' 

 acts like a rasp, and is employed in scraping or tearing away 

 portions of the substance on which the animal is feeding. By this 

 action the teeth are gradually 

 worn away in front, but this 

 is of no consequence, for the 

 lingual ribbon is always growing 

 forwards, the worn material 

 being replaced by new growth 

 behind. 



The arrangement and form 

 of the teeth are characteristic 

 and important ; and since they 

 afford one of the means by which 

 we may trace the natural affini- 

 ties of similar species, they will 

 be frequently referred to when 

 dealing with the principles of 



classification. For this reason the student should be prepared to 

 examine the lingual ribbons of molluscs with the aid of a compound 

 microscope as occasion requires. As a rule the ribbon is easily 

 stripped away from the floor of the mouth ; and, if placed in a drop 

 of water and covered with a cover-glass, the teeth are readily 

 observed. Until a little experience has been gained the observations 

 may be confined to some of the larger species, in which the ribbon 

 is both large and easily obtained. In the common whelk, for ex- 

 ample, it often measures more than an inch in length. 



It is difficult to understand how the univalve mollusc manages 

 to glide along so rapidly and gracefully on its expanded foot when 

 we observe it from above, but the difficulty is cleared away when 

 we see it creeping on the side of a glass aquarium, or when we 



FIG. 158. A PORTION OF THE 

 LINGUAL EIBBON OF THE WHELK, 

 MAGNIFIED J AND A SINGLE BOW 

 OF TEETH ON A MUCH LARGER 



SCALE 



6, medial teeth : a and c, lateral teeth 



