178 FEESH-WATEE AQUAEIA. 



orescent-shaped. THs snail lays about seven eggs and in- 

 closes them in an orbicular oapsxde. The fry are hatched in 

 from ten to fourteen days. 



Planorbis glaber is found in widely different parts of 

 Britain, but it is very local. It is much like P. albws, but 

 is smaller, more glossy, and without the raised spiral striae. 

 The animal is described as having "a yellowish- grey body;, 

 tentacles rather short, cylindrical, and ending in a blunt point ; 

 foot rather broad, especially in front, with a yellowish edge." 

 The shell is convex above, with a slight depression in the 

 middle, and concave below. It is of a brownish horn-colour, 

 and slightly transparent. There are about four compact 

 whorls, and the periphery is round. The aperture is nearly 

 round, and the sutures are rather deep. This snail has been 

 likened, when young, to the Valvata cristata. 



Planorbis nautileus is a very beautiful little snail, taken on 

 water-plants in the lakes, ponds, and ditches of many parts of 

 Britain. It is generally found upon the under-side of broad, 

 floating leaves, such as those of the Pond Weeds {Potamogeion) 

 and the Water-Lilies, but especially upon the former. The 

 (body of the animal is greyish and is spotted minutely with 

 black. The shell is about ^gin. in diameter, and is quoit- 

 shaped. There are three whorls, the body- whorl being larger 

 than the rest of the shell. The periphery is bluntly keeled. 

 This small snail lays from three to six eggs, and incloses 

 them in an orbicular capsule. The fry are hatched in about 

 twelve days. 



The Physee connect the Planorbes with the Limnaeadse. 

 There are only two species of the genus Physa in Great 

 Britain, but both are suitable for the aquarium. The Physse 

 can be easily recognised by their sinistral shells. Sinistral 

 shells are those whose coils instead of winding from left to 

 right — i.e., following the course of the sun — wind from right to 

 left, the way an ordinary screw is withdrawn. 



Physa hypnorum is found on water-plants in slow-running 

 streams and ponds throughout Britain. It is an active snail, 

 and very interesting in the aquarium, especially as a "thread- 

 spinner." The body of the animal is generally dark grey, 



