226 



LIFE BY THE SEASHORE. 



among the great blades of Laminaria. Put your specimens 

 in a glass jar filled with clean water, and examine the lower 

 surface (see Fig. 66). Some of the points of structure we 

 have already noticed : the muscular foot in the centre, used 

 here, as in many Molluscs, as a creeping surface ; the head, 

 separated from the foot by a constriction, and bearing mouth, 

 horns or tentacles, and eyes; the mantle-fringe, or flap, 

 hanging down at the sides of the body like a frill, and 

 secreting the conical shell above. In the tortoise-shell, but 

 not in the common limpet, there is a single plume, or gill, 

 exserted when the animal walks. These points studied, 

 drop your specimens for a few minutes into hot water or 

 spirit, and then remove the shells by slipping a sharp-pointed 

 knife round the sides of the animal. Detailed dissection is 

 not easy, but some points can be readily made out. Notice 

 that the mantle is arched in the 

 head region, so that it there forms 

 the roof of a small chamber 

 the mantle-cavity, which in the 

 tortoise-shell limpet contains the 

 gill. The mantle-cavity is a very 

 important structure, and you 

 should take pains to assure your- 

 self that it is outside the body- 

 cavity, that it is equivalent to 

 the gill-chamber of the Crustacea, 

 and is formed by the downgrowth 

 of the mantle-flap, a free fold of 

 skin. One other structure is of 

 great importance; this is the 

 so-called tongue, or radula, a long, 

 brownish thread, much longer 

 than the animal, which lies folded up at the right side, and 

 is very easily found. When examined with the lens it will 

 be found to be covered with numerous rows of small teeth. 

 By means of it the limpets mow down the sea-grass upon 

 which they feed, but the carnivorous Molluscs use it as a 

 drill to perforate the shell of other Molluscs. 



One other point must be noticed in regard to the anatomy 

 of the limpet. The posterior opening of the food canal, 

 instead of being at the end of the body, as one would 



rr g 



FIG. 66. Under surface of com- 

 mon limpet (Patella vulgatd). 

 mo, mouth; ma, mantle pro- 

 longed into fine processes ; 

 /, foot; g, respiratory region 

 of mantle. 



