NATURAL HISTORY OF BHOMPSTON POND. 213 



floating pond- weed. Unlike many leeches it constructs no 

 cocoon for its eggs, but instead it carries them on the under 

 surface of its body, to which the newly-hatched young adhere 

 by their posterior suckers. Its internal structure is most 

 easily examined by embedding in wax, after staining, &c., 

 and cutting transverse sections at various levels. The crop, 

 with its diverticula, its masculature, and other organs, can 

 then be studied with advantage. 



Passing now to the Mollusca, the Lamellibranchiata, 

 characterised by having four plate-like gills, is represented in 

 this pond by at least two genera, namely, Sphaerium and 

 Pisidium. In the former (fig. 42) the oval shell has two 

 equal valves, and the external ligament is posterior to the 

 hinge. The latter is distinguished by its smaller size, and by 

 the shell being less equilateral, with the backs towards the 

 anterior end. 



Among the Gasteropoda, Limnaea palustris (fig. 43), with 

 a long spiral brownish horn-coloured shell and a dark grey 

 body, bearing two pointed contractile tentacles (a), is 

 extremely common on the floating pond- weed (Potamogeton). 

 The foot is notched in front (b) and rounded behind, and the 

 black eyes are situated on small tubercles (c). The respira- 

 tory orifice, placed on the right side, is clearly visible to the 

 naked eye when the animal comes to the surface to breathe. 

 There are two generative orifices on the right side. If the 

 animal is first killed, and then immersed in caustic potash, the 

 odontophore (lingual ribbon) is easily separated from the 

 rest of the body, and can be stained with picric acid. If then 

 examined under a high power, it will be seen to consist of 

 thousands of teeth arranged in rows. When the front teeth 

 are worn away they are replaced by others from behind. 

 Limnaea deposits its eggs in a cylindrical capsule containing 

 about sixty in number (fig. 44). 



A small form, Ancylus (fig. 45) is found adhering like a 

 limpet to pieces of wood, &c. The body is grey, the foot 

 brownish and rounded in front and widened at the sides. 

 The limpet-shaped shell is of a pale horn colour. 



