260 Adjustment to Conditions of Aquatic Life 



FIG. 1 60. Limpet-shaped 

 animals. At right the larva 

 of the Parnid beetle, Pse- 

 phenus, known as the 

 "water-penny." At left, 

 the snail, Ancylus. 



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 



^B ^ have to meet and withstand the 



mmiP* : Si pi II wa "ter also, but only on one side; 



H and the form is half of that of 



|^ it, S our diagram (fig. 1 53) . It is that 



^1 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 369, 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. 1 60) 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 FlG - J 61 - T. he larva 



., , , \ 11- , i 1 of the caddis- worm, 



confusa (ng. 1 6 1 ) is also limpet-shaped ; Ithytrichi 



