in certain Aquatic Animals. 341 



cilia is the same, but as the gills in that animal differ in some 

 points from those of the salt water mussel, I have not yet been able 

 to satisfy myself as to the mode in which the water is conveyed to 

 the excretory orifice. The current on the surface of the gills re- 

 sembles that of the salt water mussel, except that on the outside of 

 the external gill it is directed from the margin to the base. It is 

 obvious that, in the bivalve mollusca, the property of exciting mo- 

 tions in the water may serve other important purposes besides re- 

 spiration, and it is probably in this way that their nutriment is 

 carried to the mouth, and tnat the ova are excreted, or conveyed 

 from one part of the body to another. 



Of the Gasteropodous Mollusca, I have examined species be- 

 longing to three of the great divisions in which Cuvier has arrang- 

 ed them, according to their respiratory organs. The Doris and 

 Eolis of the Nudibranchiata, the Buccinum undatum and other 

 species of the Pectinibranchiata, and the Patella and Oscabrion, 

 {Chiton, L.) which form the two genera of the Cyclobranchiata. 

 In all these, I have found like currents along the surface of the 

 gills and other parts of the body, directed in general in such a way 

 as to expose the respiratory surface as completely as possible to the 

 influence of the water. 



The only animal among the Mollusca in which I have not been 

 able to perceive a current, is the Ascidia, but the observation may 

 perhaps be inconclusive, as the specimens had been some time out 

 of the water. I have not yet been able to procure live specimens 

 of the Cephalopoda. 



I also found this phenomenon in the Annelides ; the animal ex- 

 amined was a species of Amphitrite, which has the gills in the 

 shape of tapering filaments, placed in two rows along the back. 

 Here the current runs forwards on the back between the two rows 

 of gills, then along the surface of the latter, which are beset with 

 tufts of cilia. In the class of radiated animals, similar appearances 

 are presented by the Actinia. 



I have not as yet been able to discover any thing analogous in 

 fishes. It seems to me, however, not improbable, that the exter- 

 nal gills which belong to the foetus of some cartilaginous fishes, and 

 which have been compared to those of the tadpole, may resemble 

 them also in this particular. 



I have not yet extended the inquiry to warm-blooded animals. 

 It is true I have made one or two observations on the chick during 

 incubation, in order to ascertain whether its respiratory process is 

 accompanied at an early period with phenomena like those I have 

 been describing. These trials have hitherto afforded no decisive 

 result. But there are other processes in the animal economy, m 

 which a similar agency may possibly be in operation. Of these 

 perhaps the most striking is the motion of fluids through canals, 

 under circumstances in which it cannot well be ascribed to a con- 

 tractile power in their coats ; but it would be premature- to i^ason 

 further on this subject at present. - - -^ '^ - -'- „''* ' ' ' "- 



