24 LIFE BY THE SEASHORE. 



on the tortoise-shell limpet, I attempted on several occasions 

 to get assistance in collecting specimens. The result was, 

 however, invariably that I was presented with young speci- 

 mens of the common limpet, with the assurance that these 

 were exactly the right thing. 



The differences are nevertheless well marked. In the 

 common limpet the thick shell is marked with ridges 

 which project at the margin of the shell; in the other the 

 surface of the shell is perfectly smooth, and marked with 

 a beautiful "tortoise-shell" pattern in brown. In the 

 common limpet the inside of the shell is glassy smooth 

 and transparent; in the tortoise-shell it is opaque white, 

 except for an elongated brown mark in the upper part. 

 Between the animals themselves the differences are much 

 more marked, as will be readily seen by putting both into 

 a glass bottle and allowing them to crawl up the side. In 

 the flattened creeping sole or foot, in the pendent fringe 

 or mantle-skirt surrounding this foot, in the horns or ten- 

 tacles at the sides of the prominent mouth, there is marked 

 resemblance; but in Patella the side of the mantle next 

 the foot is pleated and vascular, forming the breathing 

 organ of the animal, while as the little Acmcea moves you 

 will see it protrude in front a single plume-like gill. As it 

 creeps up the glass, also, you will notice that its mantle is of 

 a delicate green colour, while that of the common limpet is 

 dull-coloured ; the whole animal has also a delicate trans- 

 lucency beside which the common limpet seems coarse and 

 ungainly. 



In habitat there is also a marked difference. At low tide 

 the common limpet is found far above the water level, with 

 its shell embedded in a slight excavation of the rock into 

 which it closely fits; the tortoise-shell, on the other hand, 

 is rarely found except in pools. The little pits which the 

 common limpet makes and inhabits, together with its tre- 

 mendous power of adhesion, must diminish the evaporation 

 of moisture, and therefore diminish the risk of drying up ; 

 the thick shell probably also aids in the retention of the 

 necessary water. If you knock a living limpet off the rock 

 you will find that the under surface is abundantly moist, 

 while the specimens which have been knocked off by the 

 birds and left foot upwards seem to dry directly. The 



