2E2 LAKES AND RIVERS, 



the sexes united in one, but " require fertilization by 

 another individual." Those species which are aquatic 

 require atmospheric air, but in Britain three-fourths 

 of this order are terrestrial. Most of them are herbi- 

 vorous. The principal freshwater genera are Limnaa 

 and Planorbis ; in the latter the body is long, and, in 

 correspondence with the shell, twisted in a flat coil. 

 The tentacles are very long, the foot short, attached 

 to the body by a stalk. The shell corresponds 

 in form to the body. The animal emits when 

 irritated a red fluid. It has the vital organs on 

 the left side, and the spire of its body is coiled in the 

 opposite direction, i.e., from left to right. The form 

 of the shell resembles an Ammonite. The body is 

 much smaller than the shell. Some of the smaller 

 species of Planorbis shut themselves up in their shells 

 during the dry weather until the return of rain. They 

 all frequent stagnant or slow-running water. The eggs 

 are in a globular bag, fixed to a stone or stalk of a 

 plant. As they are sluggish the shells are apt to become 

 encrusted with mineral or vegetable deposit. P. lineatus 

 (Walker) has the body reddish-brown or purple, with 

 black speck eyes, small foot broad. The shell is quoit- 

 shaped, the upper more convex than the lower side. 

 It has 4 whorls ; inside the last whorl are rows of 

 curved plates, arranged on each side across the spire. 

 The length is 0*65 ; the breadth 0-2. P. nitidus 

 (Miiller) is shaped like the last, but flatter, and with 

 more of the spire visible. The shell is thin, glossy, 

 and prismatic, and of a light yellowish-horn, or gray, 

 and sometimes reddish colour. The epidermis is 

 very delicate ; the whorls are 4 or 5 ; the last covers 



