Dr. Sharpey on the Mechanism of Respiration 339 



is obviously intended for the purpose of renewing the water requir- 

 ed for the respiration and nutrition of the animal; but though it is 

 now a well established fact in the history of the mussel, the me- 

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

 satisfactorily. Some have contented themselves 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 in- 

 tervals, 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 cur- 

 rents in the tadpole, it struck me that the entrance and exit of the 

 water in the bivalve moUusca 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. Hav- 

 ing cut off a portion of the gill, I found that a current was excited 

 along its surface in a determinate direction, and that it moved it- 

 self 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 conveyed rapidly backwards in the interior 

 of the gill between the two layers, and almost immediately escapes 

 at the excretory orifice, or that from which the general current 

 already mentioned is observed to come out. That portion of the 



side from those of the left. Each consists of two layers, which are made up of 

 vessels set very close to one another like the teeth of a comb, across the direction 

 of the gill, and perpendicular also to the great vascular trunks with which they 

 communicate, which run along the base. In the common mussel, the two layers, 

 of which each gill is composed, are connected together at its margin, and by a few 

 points of their contiguous surfaces ; but, at the base, only one of them is fixed, 

 the other terminating at this part by a round unattached border, (fig. 4. e, e,) 

 under which a probe can be passed into the interior space between the two layers. 

 Besides the gills, the same animals are furnished with four triangular laminfB, 

 placed round the mouth, which have been called labial appendages, tentacula, or 

 accessory gills, and which probably serve more or less for respiration. 

 " 3Ianuel de Malacologie, &c. Paris, 1825. p. 157. 



VOL. II. 2 X 



