538 ORGANS OF RESPIRATION. 



and their temperature lower than in most of the active, air- 

 breathing articulated tribes. As the development of vibratile 

 cilia, on the respiratory apparatus of animals, is generally 

 inversely proportioned to the other means of activity pos- 

 sessed by these aerating organs, they are most developed, most 

 numerous and constant, and most extensively distributed on 

 the branchial apparatus of the languid and inert molluscous 

 classes, where they were already observed and described on 

 the branchiae of conchifera, before the time of Leuwenhoek. 

 These minute vibratory organs not only cover the filaments 

 of the branchiae, and line the whole respiratory cavities, but 

 are seen also on most of the naked parts of the exterior, and 

 even in the alimentary canal, as in many lower invertebrata ; 

 and the involuntary character of their movements was known 

 to Leuwenhoek, who saw them moving on pieces of the 

 branchiae of the common muscle, mytilus edulis, detached 

 from the body. The respiratory currents of tunicata, when 

 their body is perfectly motionless, were known to Cavolini, 

 who justly compared those of ascidia to the currents which 

 bring food to the polypi of zoophytes ; and Basterus referred 

 to the ciliary movements of the branchiae extending a little 

 beyond the edge of the shells, the spontaneous motions 

 which he often observed in newly hatched embryos of the 

 oyster. 



The tunicata breathe by reticulate branchiae, occupying 

 the interior of a large thoracic cavity, which receives the 

 ciliary currents by the large respiratory orifice, and expels 

 the water by the common vent of the branchial, the alimen- 

 tary, and the generative organs. In the long narrow cavity 

 of the salpa, they are disposed transversely in rings or spiral 

 turns, giving an anriulated appearance to that cavity, com- 

 pared to a trachea by Chamisso, as seen through the trans- 

 parent parietes of the body. In the higher forms of this 

 class, the branchiae generally constitute a more continuous 

 lining of the respiratory sac ; and the inhaled currents pass 

 through their elongated oval or quadrangular meshes, to 

 arrive at the efferent canal and exterior vent. Although the 

 elasticity of the exterior tunic of these animals is capable of 

 assisting in inspiration, and the contraction of the muscular 

 tunic or mantle, everywhere attached to its interior, contri- 

 butes occasionally to rapid and powerful expiration, yet the 



