ORGANS OF RESPIRATION. 539 



orifices are most open, and the respiratory currents are most 

 active, when aU these parts are entirely quiescent. The same 

 currents which aerate the branchiae by passing through their 

 innumerable meshes, bring also floating particles of food to 

 the mouth, placed in a recess at the bottom of the respiratory 

 cavity. 



The vibratile cilia, which alone produce these equable 

 and constant currents, I have found of minute size, com- 

 pactly disposed over the entire breadth of the reticulate 

 filaments of the gills, lining the smooth parts of the branchial 

 sac, covering even the tentacular filaments at the orifices, and 

 moving in most regular waves around all the meshes of the 

 branchiae, as around the arms of a polypus. On being irritated, 

 the ascidiee contract the mantle forcibly, and throw the 

 contained water to a distance from both orifices, the direction 

 of the respiratory currents being determined not so much by 

 valvular structure, as by the impulse of the waves of minute 

 vibratile cilia, which here, as in all other cases, continue to 

 vibrate on small pieces detached from the gills. The vibra- 

 tile cilia, in these inert and often fixed animals, thus serve to 

 aerate the venous blood of the system, before being trans- 

 mitted to the heart, and they bring food to the mouth with 

 the respiratory currents ; in the swimming species, they are 

 also, as in the ciliograde acalepha, the means of locomotion ; 

 in the luminous tunicata, as pyrosoma, they appear to be 

 connected with that remarkable property of emitting light, 

 as I have found them to be on the surface of beroe, and in 

 the ciliated intestine of some luminous annelides ; and they 

 assist in removing all excretions and foreign particles from the 

 interior of their body. 



In the respiratory organs of the conchifera, as in most 

 other organs of their body, the general plan of structure is 

 nearly the same as in tunicata, and they are very uniform 

 throughout the class. Here, as in tunicata, the mantle pre- 

 sents two apertures, a respiratory orifice and a vent, commu- 

 nicating with the branchial cavity, for the reception and exit 

 of the ciliary currents, and the buccal orifice of the alimentary 

 canal is situate at the bottom of this cavity. The vibratile 

 cilia, often of great size and bent, as in mytilus, cover closely 

 the whole surface of the branchial meshes, line all parts of 

 the respiratory cavity, extend over the mucous covering of 



