FRESHWATER MUSSELS 67 



Eventually the young animal arrives at the stage known as the 

 " glochidium." It now possesses a bivalve shell, furnished with 

 remarkable and movable teeth at the ventral extremity of each 

 valve, and with a single adductor muscle by which the valves 

 can be forcibly drawn together. The shell is lined by a mantle 

 on which are peculiar sensory organs ; and round the adductor 

 muscle is wound a sticky thread, the byssus, whose free end 

 passes out some distance beyond the shell margin. At this 

 stage, which may be found during February, March and April, 

 the animal escapes from the egg-shell, but is still retained in 

 the parental gills until the water in the neighbourhood is dis- 

 turbed by some passing fish, such as a stickleback, and possibly 

 by tadpoles. The parent then dis- 

 charges the glochidia through the ex- 

 halant siphon in long granular looking 

 cords held together by the entangled 

 byssi. Sticklebacks take a lively 

 interest in the masses of glochidia, 

 and make frequent dashes at them as 

 though to devour them. This, how- 

 ever, they appear not to do, but rather 

 spit them out in disgust. Meanwhile 

 the glochidia, thanks to their special 

 sensory organs, become aware of the FlG - 37- Posterior view of giochi- 

 presence of the fish, and maintain a dium ' '' byssus; '' she teeth ; 



s, sensory organs. 



rapid snapping to and fro of their 



valves. Their sticky byssi have become attached to the fins 

 and tail of many of the sticklebacks, and the fish swim away, 

 trailing behind them strings of glochidia. Sooner or later 

 many of the glochidia succeed in grasping a piece of the fish's 

 skin between the shell-teeth, and, having accomplished this, they 

 never loose their hold. The irritation set up in the skin of the 

 stickleback causes a blister to form around the giochi dium, 

 further securing it from falling off. For the next three months 

 the glochidium lives as an external parasite upon the fish, 

 nourished by the fluid which fills the cavity of the blister. During 

 this time profound changes take place in the tissues of the para- 

 site, resulting in the disappearance of the byssus, whose work 



