14^ DEVELOPMENT OF GLOCHIDIUM 



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 convenient 

 ' 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 Glochidium throws 

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

 are violently closed and again opened with astonishing rapidity 

 for 15—25 seconds, and then the animals appear exhausted and 

 lie placid with widely gaping shells — unless they chance to have 

 closed upon any object in the water (e.g. another Glochidium), in 

 which case the valves remain firmly closed." 



In about four weeks after the Glochidium has quitted its 

 host, and the permanent shell has made its appearance within 

 the two valves of the Glochidium, 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 wdiich he 

 experimented had a strong dislike to Glochidium as an article 

 of food. Sometimes a fish would taste it "just to try," but 

 invariably 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 Glochidium 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. Blocliinann, Ueber die Entwickeluug von Neritina fluviatilis Miill. : Zeit. 

 wiss. Zool. xxxvi. (1881), pp. 125-174. 



