338 in certain Aqnatic Animals. 



tends farther back. A lateraV current then appear s^ commencing 

 on each side of the head, and running in the same direction, but 

 not extending so far as the inferior one. At a period somewhat 

 more advanced, but still some time before exclusion from the egg, 

 the animal presents all these appearances in a considerably greater 

 degree. 



The next animals in which I have found the power of producing 

 currents, are the Mollusca. Among these it prevails to such an 

 extent, and exists in species differing so much in other respects, as 

 almost to lead one to suspect that it may belong to at least all 

 the animals of this class that respire by means of gills. Indeed I 

 am at a loss to conceive why so general and obvious a phenomenon 

 should have escaped the attention of naturalists, unless perhaps be- 

 cause such as have been engaged in studying these animals have in 

 most cases made their examinations on dead specimens. 



In the Mollusca, as in the larvae of the Batrachia, the currents 

 appear chiefly to serve the purpose of renewing the water on the 

 surface of the respiratory organs ; but, though agreeing in every 

 other circumstance of importance, there is one peculiarity in the 

 Mollusca, which, notwithstanding every endeavour, I have not 

 been able to discover in the Batrachia. The peculiarity consists 

 in this, that the surfaces along which the currents are excited are 

 beset with innumerable cilia, visible only with a glass, which are 

 in constant motion. In this they more or less resemble certain 

 zoophytes and infusory animalcules, in which like cilia have been 

 detected. 



The remarkable circumstances attending the respiration of the 

 testaceous acephala or bivalves, first led me to suspect that a pro- 

 perty more or less resembling what I had met with in the Batra- 

 chia, might also exist in this class of animals. I accordingly began 

 the examination with the common muscle, and, though I subse- 

 quently found the same property in others of the acephalous and 

 gasteropodous IVIollusca, yet, as I have observed the phenomena 

 with most attention in the muscle, and, as it can be easily procured 

 by any one who is desirous of inspecting the appearances in ques- 

 tion, I shall confine my description in a great measure to that spe- 

 cies alone. 



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 occasioned by the water enter- 

 ing at the posterior or large extremity of the animal, into the ca- 

 vity in which the gills are lodged, and coming out near the same 

 place by a separate orifice,* in a continued stream. This current 



" It will be recollected, that the respiratory organs or gills of the bivalve Mol- 

 lusca are in the form of leaves, of which there are two on each side, inclosed be- 

 tween the lobes of the mantle. Between them are interposed what is called the 

 foot and the prominent part of the abdomen, which separate the two of the right 



