HEDGE AND DITCH 163 



natural history are the writers who give two names to the 

 same thing !) has a double shell, thin and semi-transparent, 

 almost circular in side-view, but strongly biconvex when 

 seen edgewise. The general structure of the animal within 

 the shell is similar to that of the pond-mussel (Anodon), 

 so well known in biological laboratories, but differences 

 will be discovered by dissection. In Cyclas there is only 

 one pair of gills ; every individual is both male and female ; 

 and there are two siphons which can be protruded from 

 the shell, one serving for the entrance and the other for 

 the escape of a stream of water. When a Cyclas is kept 

 for a few hours in a dish of muddy water the siphons are 

 sometimes protruded. Any naturalist who has a micro- 

 scope and knows his Anodon pretty well, will find Cyclas 

 an interesting object of study. The animal is so trans- 

 parent that most of its structure can be made out by re- 

 moving the shell and turning the soft parts over with a 

 needle ; indeed nearly everything can be seen in a young 

 Cyclas removed from the gill of its parent, and studied 

 alive and uninjured with a low power. I have seen the 

 heart beating and the otoliths vibrating in such a specimen. 

 As in Anodon , the young are hatched in the gill during the 

 winter-months ; at this season the Cyclas is often torpid 

 and buried in the mud. 



Here is a question for students who love biological 

 problems ; it is not exactly an easy one, but soluble by 

 anybody who has a fair knowledge of animals and some 

 thinking power. Most bivalve mollusks are marine, and 

 produce multitudes of eggs ; Cyclas, which is a freshwater 

 bivalve, produces very few ; Anodon, which also inhabits 

 fresh waters, produces very many. How can these facts 

 be explained ? (12) 



In summer a Cyclas will protrude its broad fleshy foot, 

 and move about freely ; occasionally rising to the surface 

 of the water and creeping on the surface-film as on a ceil- 

 ing. A young Cyclas can climb much better than a full- 

 sized one, and it is chiefly young ones which rise to the 



