172 FEESH-WATEB AQUAEIA. 



on the right side of the mantle. When this snail is young 

 it is either male or female, but when it is old it becomes both 

 male and female. It is found in ponds, ditches, canals, 

 and slowly-running streams nearly everywhere. The body of 

 the animal is rather transparent, of a yellowish-grey colour, 

 and spotted with white. The tentacles are rather thick and 

 placed near together. The eyes are large and black, and 

 situated upon very short pedicels at the base of the tentacles. 

 The shell is about Jin. long and of a yellowish horn-colour; 

 its shape is conical, and there are five whorls, the largest 

 of which more than equals in length all the rest together. 

 The sutures are deep; the aperture is circular and the 

 operculum round and horny. The shell is found attached to 

 the cases of certain caddis-worms. The V. piscinalis lays 

 from sixteen to • twenty eggs, which are hatched in about 

 the same number of days. The shells of the fry are so 

 transparent that the young animals within can be very 

 plainly seen. 



Valvata cristata, though not large enough for the ordinary 

 aquarium, is interesting in those small tanks in which fish 

 are not kept. It is found upon weeds growing ia slowly- 

 running or stagnant water. Like the last species, and as its 

 specific name denotes, it possesses a branchial plume and a 

 branchial thread. Its body is dark brown, slightly spotted 

 with black. The tentacles are close together and thread-like. 

 The shell is so much compressed that it somewhat resembles 

 that of the genus Planorhis. It is about |-in. in diameter, 

 and of a Kght horn-colour. The operculum has been likened 

 to an " inverted pot-lid." 



Of the six families of British molluscs belonging to the 

 order Pulmohranchiata, only one of them is aquatic — lAm- 

 nseidse ; and as all the members of this family are pulmo- 

 branchiate, it is necessary that they should occasionally 

 come to the surface of the water for atmospheric air. 

 Some of them, by ascending the stalks of the water-plants 

 and the like, even leave the water altogether for a time. 

 The Limnasidx are divided into the following four genera: 

 (1) Planoriis ; (21 Physa; (3) Limnsea ; (4) Ancylus. All the 



