MOLLUSCS. 



307 



occurs. Otina is not represented in the United States. The shells are ear-shaped, 

 and colored, and the animals have the same method of locomotion as Pedipes, living 

 among rocks between tide-marks. 



All of members of the Limn^id^ or pond-snail family are inhabitants of fresh 

 water, and, so far as investigations show, are most numerous in temperate regions. 

 Over six hundred nominal species exist, which belong largely to the genera Planorhis, 

 Physa, and Limnwa. The shells are very variable in shape, but, with but little expe- 

 rience, one readily recognizes the members of the family. In some, the shell is a long 



Fig. 354. — Different forms of lAmnosa elodes, 



spiral, others have it coiled like a bit of tape, while still others have a shell without 

 trace of spiral, but flattened and limpet-like. Each of these three types of shell is 

 regarded as affording characters of sub-family rank, and we have the Limnaein£e, 

 Planorbinee, and Ancylinse respectively. 



The pond-snails are exclusively vegetable feeders. They frequent the still waters of 

 sluggish streams and ponds, their thin shells being poorly adapted for a life in rapidly 

 running creeks. They usually require to go to the surface to breathe, but, as noticed 

 on a previous page, this is not always the case. The eggs are laid in clusters attached 

 to sub-aquatic objects and imbedded in a gelatinous matrix. Frequently specimens 

 may be seen progressing at the surface of the water shell downwards, the bottom of 



Fig. 365. — lAmnaia stagnalis, pond-snail. 



the foot being just above the liquid. An interesting fact, first pointed out by Pro- 

 fessor E. Ray Lankester, is that in this family the velum persists in the adult, nearly as 

 large relatively as in the veliger stage. Though these forms have been much studied, 

 this fact escaped observation until 1883. 



First in order comes the genus Limncea with a thin, horn-colored, slender, spiral 

 shell and a large aperture. Important in separating this from the next genus to be 

 mentioned is the fact that the shell is always dextral, that is, the whorls revolve from 

 left to right. In times of drought the Limneans burrow into the mud and close the 



