162 



On the Parasitic Nature of the Fry o/Anodonta cygnea. 

 By the Rev. W. Houghton, M.A., F.L.S. 



The fresh-water Mussels, or the family of the Unionida, 

 have received a considerable share of attention from zoolo- 

 gists. The facility with which the different species may be 

 obtained, and the highly interesting embryonic forms of these 

 molluscs, whether during the time they remain within the 

 folds of the external branchiae, or after they have been ex- 

 cluded, render them favorite subjects for investigation. The 

 whole question of the development of the fry is one of con- 

 siderable interest ; but it appears that all investigations that 

 have hitherto been published on this point relate to their 

 intra-branchial existence, while we are left entirely, I believe, 

 without information on the condition and development of the 

 young Anodontas during the period that elapses between their 

 exclusion and their assumption of those characters that belong 

 to the mature animal. The young Anodonta, at the time it is 

 ready for exclusion, bears a form very different from that 

 which it is destined ultimately to assume. This, no doubt, 

 holds good in the case of the other Bntish members of the 

 group ; it is certainly true of Unio pictorum, Avhich, however, 

 appears to be later than the Anodonta in parting with the fry. 

 So different, indeed, is the form of the young animal during 

 its intra-branchial state, that M. Rathke,^ in 1797, actually 

 regarded it, not as the young mollusc, but as a veritable 

 parasite, to which he gave the appropriate name of Glochi- 

 dium {yXwj^ig), in aliusion to the two curious serrated points 

 or hooks which it possesses. Rathke's opinion was endorsed 

 by M. Jacobson, in a paper published in the Danish ' Trans- 

 actions' in 1828. t But the real nature of the bivalves found 

 in the branchiae of Anadonta and Unio was fully proved by 

 C. G. Carus in 1830.J In his valuable memoir on this sub- 

 ject, which may be regarded as the fountain-head of all that 

 is really known concerning the anatomy of the Glochidium- 



* * Naturhistorie Selskabets Skrifter/ torn, iv, i, p. 139. Copenhagen, 

 1797. 



•j- "Undersogclser til naemere Oplysning af den herskende Mening on 

 Damuslingernes Fremailing od Udvikling." Translated in the Memoirs of 

 the Prench Academy, under the title of " Observations sur le Developpement 

 pretendu des (Eut's des Moulettes ou Uuios et des Anodoutes dans leur 

 branchies." 



J " Neue Uutersuchungen iiber die Entwickelungsgescliichte unserer 

 Fliissmuschel." 'Nov. Act. Nat. Cur.,' torn, xvi, part i, p. 3. 1831. 



