260 Adjustment to Conditions of Aquatic Life 



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animals. 



Limpet-shaped 

 At right the larva 

 of the Parnid beetle, Pse- 

 phenus, known as the 

 "water-penny." At left, 

 the snail, Ancvlus. 



There are other more numerous forms living in rapid 

 water that cling closer to the solid surfaces, move about 

 upon and forage freely on these surfaces, and the 

 adaptations of these are related to the surfaces as much 



as to the open stream. These 

 have to meet and withstand the 

 water also, but only on one side; 

 and the form is half of that of 

 our diagram (fig. 1 53) . It is that 

 figure divided in the median 

 vertical plane, with the flat side 

 then applied to the supporting 

 surface, and flattened out a bit at 

 the edges. This is not fish form, 

 but it is the form of a limpet. 

 This is the form taken on by a 

 majority of the animals living in rapid waters. When 

 the legs are larger they fall outside of the figure, as in 

 the mayfly shown on page 367, and 

 are flattened and laid down close 

 against the surface so as to present only 

 their thin edges to the water. When 

 the legs are small, as in the water- 

 penny, (fig. 160) they are covered in 

 underneath. Sometimes there are no 

 legs, as in the flatworms, and in the 

 snail, Ancylus. 



Here, surely, we have the impress 

 of environment. Many living beings 

 of different structural types are mould- 

 ed to a common form to meet a com- 

 mon need; and even the non-living 

 shelters built by other animals are 

 fashioned to the same form. The case 

 of the micro-caddisworm, Ithytrichia 

 confusa (fig. 161) is also limp^ -' aaped; 



Fig 



161. The larva 

 of the caddis- worm, 

 Ithytrichia confusa. 



