their Respiration. 109 



" When a live muscle (Mytilus edulis) is attentively examined 

 in a vessel of sea water, it is soon observed to open its shell 

 in a slight degree, and about the same time a commotion 

 may be perceived in the water in its vicinity. This is occa- 

 sioned by the water entering at the posterior or large ex- 

 tremity of the animal, into the cavity in which the gills are 

 lodged, and coming out, near the same place, by a separate 

 orifice, in a continued stream. This current is obviously 

 intended for the purpose of renewing the water required for 

 the respiration and nutrition of the animal ; but, though it is 

 now a well-established fact in the history of the muscle, the 

 mechanism by which it is produced has not, so far as I know, 

 been satisfactorily explained. Some have contented them- 

 selves with ascribing it to an alternate opening and shutting 

 of the shell ; but, as no such motion takes place in the shell, 

 except at distant and irregular intervals, it is evident that the 

 constant passage of the water cannot be explained in this 

 way. Others, who saw the insufficiency of this explanation, 

 have endeavoured to account for it by assuming peculiar 

 contractions and dilatations of the mantle in virtue of its 

 muscular power, or, like M. de Blainville, have supposed 

 that the triangular labial appendages placed round the mouth 

 excited the current by their constant motion. After meeting 

 with the currents in the tadpole, it struck me that the entrance 

 and exit of the water in the bivalve Mollusca might not 

 improbably be owing to a similar cause ; and that the surface 

 of the respiratory organs, and other parts over which the 

 water passed, might have the power of exciting currents in 

 it, the combined effect of which would give rise to the entering 

 and returning stream. 



'' This conjecture proved, on actual examination, to be 

 right. Having cut off a portion of the gill, I found that a 

 current was excited along its surface in a determinate di- 

 rection, and that it moved itself through the water in an 

 opposite one, exactly as in the case of the tadpole. The 

 whole surface of the gills and labial appendages or accessory 

 gills, the inner surface of the cloak, and some other parts, 

 produced this effect. The currents on the gills are of two 

 kinds. When finely powdered charcoal is put on any part 

 of their surface, a great portion of it soon disappears, having 

 penetrated through the interstices of the vessels into the 

 space between the two layers of the gill. On arriving here, 

 a part is forced out again at the base of the gill from under 

 the border of the unattached layer, but most of it is con- 

 veyed rapidly backwards in the interior of the gill between 

 the two layers, and almost immediately escapes at the ex- 



