REPRODUCTION AND ARTIFICIAL PROPAGATION OF FRESH-WATER MUSSELS. 1 55 



REACTIONS OF HOOKED GLOCHIDIA. 



The larvae of Symphynota complaiiaia, which are provided with stout hooks and 

 as a rule find permanent lodgment only on the fins and other external parts of the fish, 

 were used in studying the reactions of the hooked type of glochidium. In several re- 

 spects they differ from the hookless forms. When removed from the marsupium and 

 placed in water, they exhibit spontaneous contractions which occur at irregular and 

 rather long intervals, and this irritability may continue in the laboratory for a day or 

 two, or until the glochidia begin to disintegrate. Under such conditions the valves 

 are only partially closed at each contraction of the muscle, which, moreover, is never 

 strong enough to bring the points of the hooks into contact. It is followed at once by 

 relaxation of the muscle and the shell remains widely open until the next snap occurs. 



Hooked glochidia, in striking contrast with the behavior of the hookless forms, 

 respond very actively to tactile stimuli, and, as has been stated, close completely and 

 immediately when touched with any object. This reaction must be the main factor 

 in bringing about their attachment to the fish's fins, when they are brushed over by 

 the latter while lying on the bottom. With glochidia like those of Symphynota com- 

 planaia the mere contact is sufficient to produce complete closure of the valves, and, 

 whether they are exposed to the fish's blood or not, attachment is possible as a result 

 of the tactile stimulus alone. They do react to blood, however, and exhibit a few 

 successive contractions, from 5 to 15, before final closure, but the way in which the 

 response occurs is quite different from that shown by hookless glochidia under similar 

 conditions. Instead of being thrown into violent and rapid snapping, the valves closing 

 and opening alternately, there is only partial recovery after each contraction, while 

 the valves are brought closer and closer together by a series of short jerks. The final 

 act of closing is interesting. As soon as the points of the hooks touch, the contraction 

 of the adductor muscle becomes continuous and the hooks are slowly bent inward 

 against each other. Under the steady pressure exerted by the muscle, aided probably 

 by the action of the myocytes, which have been described by Schmidt (1885b), the 

 spines on the outer surface are apposed and the hooks turned in completely between 

 the valves, the margins of which are brought together, if no object intervenes. It will 

 be readily understood that, owing to the turning in of the hooks, the spines are pressed 

 into the fish's tissues, when attachment to the host takes place, and a firm hold is thereby 



secured. 



When the glochidia of Symphynota complanata were exposed to salt solutions, the 

 contractions produced were of the kind just described. KCl, KNO3, and NH^Cl in 

 solutions of 0.5 to i per cent caused a few successive jerks, the contractions being more 

 vigorous and closure occurring sooner with the stronger solutions. NaCl and NajCjO^ 

 in the same strength acted less energetically, and it was necessary to use a 2 per cent 

 solution to produce the same effect as was obtained with the weaker solutions of potas- 

 sium and ammonium salts. A 0.5 per cent solution of CaCl^ produced no contractions, 

 while a i per cent solution after a latent period of 15 minutes caused either partial or 

 complete closure of the valves. MgClj and MgSO,, in solutions of 0.5 and i per cent. 



