GTEODAOTTLUS ELEOAKS OCCUBRING ON STICKLEBACKS. 209 



deserves little consideration, except from the necessity of noting all 

 the varieties of its form and structure, by which they are enabled 

 to fit the appropriate doors to the many empty dwellings in their 

 cabinets. 



Tlie supposition that the particular part of the foot occupied by 

 the byssus in some bivalves corresponds with that which supports 

 the operculum in Gasteropods, is just as unphilosophical as the 

 notion that the vesicular float of lantliina is an extreme modifica- 

 tion of the operculum. JSTor is the mass of byssus which closes up 

 the interval between the valves of the unattached Bysso-arca to be 

 regarded as the homologue of the operculum in Gasteropods, 

 although it may fulfil an analogous function. There are juster 

 grounds for believing that the float of lantliina, Macgillivrayia, &c,, 

 the suspensory threads of Litiopa, Planaxis, &c., the temporary 

 byssus of the young Anodon, Naia, and Cyclas, and the permanent 

 byssus of other bivalves, fixed or unattached, are all essentially 

 equivalent structures, having a local origin altogether distinct from 

 the " operculigerous lobe " as it exists in Gasteropods, or from what 

 I am induced to regard as its homologue in Conchifers. 



On the Occurrence of Gyrodactylus elegans on Sticklebacks in the 

 Hampstead Ponds, January 1860. By C.L. Bradley, Esq., F.L.S. 



[Read February 16th, I860.] 



In examining some of the common Sticklebacks obtained from the 

 Hampstead Ponds during the present month, I found them infested 

 with numerous minute parasitic worms. Although these were 

 more conspicuous upon the fins, tliey were scattered over the 

 general surface of the skin, and were attached by one end to the 

 fish, while the other floated freely. The parasite has the ex- 

 ternal characters of a suctorial Annelid. The body is subcylin- 

 drical, annulate, without setje or cilia, and terminated posteriorly 

 by a suctorial base. In its mode of progression, in the expansion 

 and contraction of the body, it closely resembles the Leech. When 

 fully stretched out, it measures about -^t^ of an inch in length, and 

 about -jl^th in breadth, tapering from the middle towards both ends, 

 the cephalic portion being by far the narrower. The latter is bifid, 

 each division furnished with a retractile brush-like extremity, 

 which the animal uses as a tactile organ and also for progression. 

 The posterior base is of a horse-shoe form ; the curved margin is 

 divided into sixteen digitations, each having an independent move- 



