MOLLUSCA. 461 



tance, the former issuing under the riglit tentaculum, the 

 latter at the puhnonary cavity. In consequence of this ar- 

 rangenifciit, the individuals of L. stagnal'is have been ob- 

 served by Geoffroy and Mulleb to unite together in a 

 chain during coition, the first and last members of the se- 

 ries exercising only one of the sexual functions, the inter- 

 vening individuals impregnating and receiving impregna- 

 tion at the same time. Whether this is the constant or 

 only accidental practice of this species, does not appear to 

 be determined. We know that many other species of the 

 genus are mutually impregnated^ as usual, in pairs only. 



The species of this genus are numerous. They reside in 

 pools, lakes, and rivers, and furnish a favourite repast 

 to the difterent kinds of trouts and water-fowl. The fol- 

 lowing are natives of Britain : — L. stagnalis, frag'ilis^ pa- 

 lustrii^ f OS sarins^ octaiifr actus, detritus, aur'icularius, 'pur- 

 trls, glutinosus, and probably Helix lutea of Montagu. 



With regard to the Lymneus auricidarius, it would ap- 

 pear, from the observations of DnAPARNAUi), " Histoire 

 des Mollusques,'''' p. 49., that it exhibits a very singular 

 structure of the respiratory organs. We shall quote his 

 own words : — " L'animal esi pourvu de quatre filamens ou 

 tubes qui partent de la partie superiure du cou, pros du 

 manteau ; ce sont des trachees. Ces tubes sont longs, 

 blancs et tres transparens, et on ne leg distingue bien qu"* a 

 la loupe. Leur surface est com me rugucuse, et leur extre- 

 mite est vm peu renflee. lis sont retractiles. L'animal les 

 fait sortir a volonte, un, deux, trois ensemble : 11 les agite 

 et les contourne sans cesse en divers sens : ce qui fait qu"'on 

 les prendi'oit pour de petits vers. Je presume que par ce 

 mouvement ces organes separent de Teau Fair que y'est 

 contenu et Tabsorbent. Cet animal est tres sujet, ainsi que 

 les autres gasteropodes fluviatiles, a etre infeste par le nais 

 ve7-micularis, qui se loge ordinairement cntre le cou ct le 



