56 LAMELLIBRANCHIA — UNIONIDAE. 



When the embryo has attained the stage of organisation just 

 described, it is ready for ejection from the mother ; on coming into 

 free contact with the water, the egg-envelope bursts and the embryo 

 emerges from it. The larva thus set free is known as the Gloehidium. 

 The embryos found by the older investigators (Rathke, Jacobson) 

 in the gills of Lamellibranchs and considered to be parasites were 

 called Glochidiv/m pavasiticum. 



The remarkable fact that the Glochidia remain for a time parasitic 

 on fishes was discovered by Leydig (No. 32) and then further in- 

 vestigated by Bkaun. F. Schmidt and Schierholz have recently 

 given a detailed description of what takes place, and we shall here 

 follow chiefly their account. The larvae, when freed from the mother, 

 become connected together in large masses by means of their glutinous 

 filaments, and in this form rest at the bottom of the water, occa- 

 sionally rotating upwards.* Chance brings them no doubt into 

 contact with fish, and a few of them succeed in attaching themselves 

 to these by the help of their shell-hooks. Unio only attaches itself 

 to the gills of fish, but the Glochidia of Anodonta, which are more 

 richly provided with hooks, may also become attached to the fins and 

 the skin. 



The hook-apparatus, according to Schierholz, is less developed in Unio, 

 and it is an interesting fact that it may be altogether wanting in certain 

 North American Unionidae (Lea). The same is the case, according to v. 

 Jhering's recent observations (No. 25), in the larvae of Soutli American 

 Unionidae, which are devoid of the tufts of setae, and perhaps also of the 

 larval filament. In these American Unionidae, therefore, the biological con- 

 ditions seem to differ somewhat from those of the European forms, and it 

 would be interesting to ascertain the conditions of parasitism in the larvae of 

 these Lamellibranchs. t The larval filament and the shell-hooks are wanting 

 in the Gloehidium of Anodonta complanata, although in other respects the 

 organisation of these larvae is the same as that of other Glochidia, and they 

 lead a parasitic life (Schierholz). 



■ [It is often stated that the Glochidia arc only discharged when fish are in 

 the neighbourhood, but Latter (Proc. Zool. Soc, 1891) found that he could 

 produce a discharge of Glochidia by gently stirring the water in which the 

 Anodons were lying. He also observed cord-like ejection of Glochidia from an 

 undisturbed Anodon in its native water. The Glochidia cannot swim, but 

 when discharged sink to the bottom, where they lie on their dorsal surfaces, 

 the thread streaming up into the water. In this position the Gloehidium lies 

 powerless to move in any direction, and here, too, it dies unless a suitable 

 " host " is brought into contact with its thread. — Ed.] 



t We do not know of any other statements upon this subject, although it is 

 not impossible that such may exist among the mass of malacological literature 

 which is difficult to review ; v. Jhkring mentions that he found Unionid 

 larvae on fish in Soutli America. 



