148 DEVELOPMENT OF GLOCHIDIUM chap 



bottom, and there remain resting on their dorsal side, with the 

 valves gaping upwards and the so-called byssus streaming up 

 into the water above them. There they remain, until a conven- 

 ient ' host ' comes within reach, and if no ' host ' comes within a 

 certain time, they perish. They are evidently peculiarly sensitive 

 to the presence of fish, but whether they perceive them by smell 

 or some other sense is unknown. " The tail of a recently killed 

 stickleback thrust into a watch-glass containing Grlochidium 

 throws them all into the wildest agitation for a few seconds ; 

 the valves are violently closed and again opened with astonish- 

 ing rapidity for 15-25 seconds, and then the animals appear 

 exhausted and lie placid with widel}' gaping shells — unless they 

 chance to have closed upon iinj object in the water (^e.g. another 

 G-lochidium)^ in which case the valves remain firmly closed." 



In about four weeks after the G-lochidium lias quitted its host, 

 and the permanent shell has made its appearance luithin the two 

 valves of the Grlochidium^ the projecting teeth of the latter press 

 upon the ventral edge of the permanent shell, at a point about 

 half way in its lengthward measurement, retarding the growth 

 of the shell at that particular point, and indenting its otherwise 

 uninterrupted curve with an irregular notch or dent. As growth 

 proceeds, this dent becomes less and less perceptible on the 

 ventral margin of the shell itself, but its effects may be detected, 

 in well-preserved specimens, by the wavy turn in the lines of 

 growth, especially near the umbones of the young shell. 



Mr. Latter found that all species of fish with which he ex- 

 perimented had a strong dislike to Grlochidium as an article of 

 food. Sometimes a fish would taste it '' just to try," but in- 

 variably spit it out again in a very decided manner. The cause 

 of unpleasantness seemed not to be the irritation produced in 

 the mouth of the fish by the attempt of the Grlochidium to attach 

 itself, but was more probably due to what the fish considered a 

 nasty taste or odour in the object of his attentions. 



The following works will be found useful for further study 

 of this portion of the subject : — 



F. M. Balfour, Comparative Embryology, vol. i. pp. 186-241. 



F. Blochmann, Ueber die Entwickelung von Neritina fluviatilis Miill. : Zeit. 



^vis.s. Zool. xxxvi. (1881), pp. 125-174. 

 L. Boutan, Recherches sur I'aiiatomie et le developpement de la Fissurelle ; 



Arch. Zool. exp. gen. (2) iii. suppl. (1885), 173 pp. 



