190 



THE FRESH-WATER MUSSEL 



[CH. VI 



inner lamellae. Shortly after reaching the glochidium 

 stage they emerge from the egg-shell and now maintain 

 their attachment to the parent by means of their byssus 

 threads. 



d 



b. 



77V 



Fig. 35. Glochidium larva, ventral view. 



Byssus thread, coiled round the adductor muscle, cut short. 

 d. Future mouth. m. The adductor muscle showing through 

 the mantle. . Sensory cells. t. Main teeth and denticles on 

 ventral margin of each valve. 



It is not until the following February or March that 

 the glochidia, measuring about 0'28 mm. dorso-ventrally 

 and 0*35 mm. antero-posteriorly, are discharged from the 

 brood-chamber into the surrounding water. The presence 

 of a fish in the immediate neighbourhood, or any other gentle 

 disturbance of the water, stimulates the parent mussel to 

 discharge her young. I have observed that sticklebacks 

 throng round a mussel when the glochidia are being 

 discharged and appear much excited by the performance. 

 They frequently snap at the entangled masses but spit 

 them out again as though distasteful. Usually the young 



