Our British Snails 29 



of univalves some are " land snails " and some 

 "water snails." It would say of itself, "I am 

 a species of the genus Unio (unio, a pearl), which 

 belongs to the family Unionidae, which belongs 

 to the sub-order Isomya {i.e. having muscles 

 of equal power to close the two valves of the shell) , 

 which belongs to the order Lamellibranchiata 

 [i.e. having gills arranged in leaf-like fashion), 

 which belongs to the sub-class Pelecypoda (i.e. 

 having a foot somewhat of an axe-shape), which 

 belongs to the class Acephala (headless), which 

 is the second of the two chief classes into w^hich 

 Mollusca are divided. 



"I differ from the Gasteropoda (whether they 

 be terrestrial or aquatic) in that I and my near 

 relations are exclusively aquatic and of a sedentary 

 life, which makes the protection of two encom- 

 passing shells necessary. These shells are secreted 

 by my mantle lobes, and are united by a ligament 

 which tends to make the valves ' gape ' for 

 water and food and by two contracting muscles 

 which close them in danger. I have a degenerate 

 brain and no eyes. My mouth has neither jaw 

 nor teeth, but possesses nervous lips covered 

 with cilia, the vibration of which carries food- 

 laden water to my mouth. My foot, when pro- 

 truded, is seen as a large muscular appendage, 

 and, by alternately expanding and contracting, 

 it enables me to burrow or plough through mud 

 or even sand, and so disturb the minute 



