308 



LOWER INVERTEBRATES. 



Fig. 366. — Limncea 

 megasoma. 



aperture of the shell with an epiphragin like that of the Helieidse, thus preventing any 

 desiccation of the fluids of the body. When the rains again till the ponds, they come 

 out of their burrows and lead a fi-ee life. Of the species but little 

 can be said, and we will let our figures sjjeak for themselves. All 

 figured are from the United States, though several are found in 

 Europe as well. Doubtless, when these forms are studied in the 

 proper manner, by rearing in confinement all of the progeny of 

 a single pair, and continuing the operation for several generations, 

 it will be found that many of the so-called species are but vagaries. 

 Indeed, Mr. P. R. Whitfield has done this with specimens of Xiwt- 

 ncea megasoma and has found that, in this way, variations were 

 produced, which conchologists, not knowing the history of the 

 group, would describe as distinct species. Mr. Whitfield's experi- 

 ments however, were not conclusive, as there was apparently a 

 lack of nutrition and a higher temperature than the normal, 

 which doubtless had an effect on the forms produced. 



Limncea is essentially a northern genus, reaching its highest 

 development in North America, in the British possessions; 

 Physa on the other hand, is more southerly. It has a thin, 

 amber-colored shell, the whorls of which revolve from right to 

 left. The species are much more active than those of Limncea. 

 The tentacles are long and slender, and the jaw is formed of a 

 single piece. Twenty-three so-called species occur in North 

 America. 



The genus Pompholyx is noticeable, from the fact that, 

 while the shell is dextral, the genitalia open on the left side 

 of the body. The shell is short and broad. Three species are 

 known, all from the Pacific region of the United States. They were formerly supposed 

 to have two pairs of eyes. 



In Planorbis the shell is wound in a flat spiral, like a roll of tape, 

 showing the whorls on either side. The animals prefer still water, 

 where they move about in a sluggish manner. About a hundred and 

 fift)' species have been described, of which about twenty-five occur in 

 the United States. Any description of the \arious forms would prove 

 tiresome reading for any but the systematic student. 

 The species of Ancyhis and Qundlacliia are shaped nearly like the limpets Acmcea 

 and Crepidula, and, judging from the shell alone, one would not realize 

 that the animals were so distinct. These fresh-water limpets have the 

 same habits as their marine equivalents. They live attached to the 

 under sides of stones below the surface of the water, feeding on con- 

 fervas and other plants. In both, the body is sinistral, the genital open- 

 ings being on the left side of the body. In Ancylus, of which we have 

 about twenty species in the United States, the shell is not at all spiral, but in Gund- 

 lachia it resembles Crepidula in this respect. Two species of the latter genus are 

 found within our boundaries, the three remaining species being from the West Indies 

 and Tasmania. 



The fresh-water pulmonates are badly infested with parasites, most of which are 

 stages of wonns which reach their complete development in some of the vertebrates. 



Fig. 357. —a, Physa ampul- 

 lacea; 6, P. elongataj c, 

 P. heterosiropha. 



Fia.SBS.—Plaiv- 

 orbis. 



Fig. 3B9. — 

 cytus. 



Aftr 



