VALVATID^. 71 



mal has another peculiar organ to facilitate its respira- 

 tion^ consisting of a filament or appendage to the mantle, 

 which might be mistaken for a third tentacle or a para- 

 sitic worm, and is placed on the right side of the body. 

 The form of the shell and operculum is very graceful, 

 and is somewhat like that of Trochus or Margarita. 



The members of this family and genus are vegetable 

 feeders. They are very shy. Miiller relates that he 

 was tantalized by watching them for several hours, in 

 the hope that they would show themselves and enable 

 his draughtsman to make a sketch of the animal and 

 its curious plume, but that he was disappointed. He 

 appears to have consoled himself by the idea that the 

 little snails acted on the proverbial principle that you 

 were not to put any trust in man ! The reproductive 

 system of these mollusks is peculiar, and resembles 

 that of Ancylus, or the freshwater Limpet. Although 

 each individual is of both sexes, it is at first only male 

 or female, and afterwards changes its sex. They are 

 gregarious, and inhabit slow rivers, streams, canals, and 

 nearly stagnant water. The shells may often be seen 

 attached to the cases of the Phryganea, or May-fly, and 

 thus collected form a very pretty object. 



The shells of some of the Valvatida closely resemble 

 in shape the cases made by the larvae of certain insects ; 

 and their similarity is so great that Mr. Swainson pro- 

 posed a new genus of Mollusca for these insect-cases, 

 under the name of Thelidomus. Such instances of mi- 

 metic analogy occur in other branches of the animal king- 

 dom. The valves of some Entomostraca, belonging to 

 the genus Estheria, are not unlike those of a young Ano- 

 donta in appearance, although their structure and compo- 

 sition are very different. 



