260 Adjustment to Conditions of Aquatic Life 



FIG. 1 60. Limpet-shaped 

 animals. At right the larva 

 of the Parnid beetle, Pse- 

 pherms, 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 

 have to meet and withstand the 

 water also, but only on one side; 

 and the form is half of that of 

 our diagram (fig. 153). 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. 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 



,, , ^ N ...... i 1 



confusa (fig. 161 ) is also limpet-shaped ; 



FlG 



1 - ,T. he larva 



of the caddis- worm, 



ithytnchia confusa. 



