468 LESLIE B. AREY 



On reflection, one is struck with a rather uniform result which 

 has been diversely gained in these larval mussels whose further 

 development is dependent on an ectoparasitic sojourn upon an 

 appropriate fish host. 



The glochidia of Unio and Anodonta, supplied with larval 

 threads, are extruded entangled in masses. The hooked but 

 threadless Symphynota larvae are spawned within a ropy mucus. 

 Strophitus and Obliquaria discharge their glochidia in tenacious, 

 gelatinous cords. Most of the remaining glochidial forms are said 

 to be more or less adherent in mucus-bound masses. In these 

 cases, practically without exception, there is a liberation of the 

 larvae to an independent condition soon after being spawned. 

 Thus, the threads of Unio dissolve immediately, those of Ano- 

 donta within a day or two; many of the Strophitus glochidia are 

 extruded from the cords within a few minutes after their dis- 

 charge; the slime of other forms is soon dissolved. Does it not 

 seem reasonable that these relations are each helpful in keeping 

 the larvae together until they have become established on the 

 bottom? Soon becoming free, they would be advantageously 

 aggregated to become whirled up by the respiratory or other 

 currents produced by fishes, attracted, for example, by animal 

 associations about a mussel bed. In this way the chances for a 

 mass infection are better than if the individual glochidia were 

 widely dispersed by currents or other causes. Such an explana- 

 tion obviates the necessity of assuming, as Howard ('14) has 

 done, that heavy infections presuppose the presence of fish at the 

 time of glochidial extrusion. Although Latter ('91) interpreted 

 his experiments to indicate that the presence of fish does not 

 induce the emission of glochidia, Howard beheves that ''they 

 support the probabihty that the approach of fish is the normal 

 stimulus in eliciting the emission of glochidia. "^ On the con- 



^ A curious relation exists between certain mussels and the 'Bitterling,' Rho- 

 deus amarus, a cyprinoid fish of central Europe. According to the original 

 account of Olt ('93), at the breeding season the genital papilla of the female Rho- 

 deus elongates to a tubular ovipositor about the same length as the fish itself 

 (60 to 80 mm.). The ripened eggs are few, but about 2.5 mm. in size; the ovi- 

 positor is introduced between the gaping valves into the mantle cavity of a unio 



