HALF AN HOUR WITH SHELL-FISH. 197 



of the limpet has long been a favourite object with 

 microscopists, as, when mounted, it shows the 

 numerous rows of teeth, amounting to more than 

 two thousand in number. The powerful manner 

 with which the limpet adheres to rocks and stones 

 has been proverbial for ages. Keaumur found that 

 a weight of nearly thirty pounds was required to 

 detach a single individual from its hold. This 

 wonderful power of adherence is brought about mainly 

 by suction, assisted by a very strong natural glue, 

 which the animal has the means of secreting. We 

 have various species of limpet in our native seas. 

 Among others may be noticed Patella pellucida, 

 obtained from deeper water, and often to be found 

 adhering to the roots of the larger sea-weeds when 

 these have been cast up. It has, as its specific name 

 implies, a semi-pellucid shell, ornamented by blue 

 lines which radiate from the apex to the margin. 

 Patella atliletica, or "Horse-limpet," is larger, and 

 has bold spiny ribs projecting down its sides. It is 

 not an uncommon shell. Nearly allied to the above 

 is the little Acmea a shell most common on our 

 northern shores. When fresh, it is beautifully 

 mottled with chestnut and white. It is, however, 

 only about half an inch in length. 



The Limpet family includes a good many allied 

 genera, among which the Chitons are perhaps the 

 most remarkable (Fig. 104). Their shells are com- 

 posed of a number of plates, slightly overlapping 

 each other, so that it curls up when the animal has 



