RESPIRATION. 253 



adult individuals. One, whose sac measured an inch and a 

 half in length, I saw respire eighteen times per minute ; and 

 the larger one, mentioned above, respired ten times per 

 minute. The time seemed to be pretty equally divided 

 between inspiration and exspiration." In those bivalves 

 whose cloak forms a shut sac, the water is sucked in through 

 the branchial tube, when the capacity of the sac is increased 

 by its own expansion, or by the opening of the shells ; and 

 by its muscular contraction, aided sometimes by the closure 

 of the shells, it is again expelled in a stream from the anal 

 siphon : but there is no regularity in the process in such 

 species as I have observed in confinement. It is the same 

 with the Tunicata. The branchial sac is muscular, and just 

 as its capacity is enlarged, apparently by the contraction of 

 its longitudinal fibres, the water flows in to fill the space in 

 a slow and uniform current, through the branchial aperture 

 only, for none can be detected entering by the anal orifice. 

 It is, after a space, expelled again by a contraction of the 

 annular fibres of the sac, but the voluntary contractions for 

 this purpose, as stated above, take place at irregular intervals 

 of time, and, for the most part, not oftener than once in a 

 minute.* 



I have told you that the respiration of the mollusca is at 

 all times slow, and easily suspended for a long period; but, 

 to obviate the inconveniences which might result from this, 

 and to supply the place of that regularly alternate and cease- 

 less play of the respiratory muscles of the vertebrates, it has 

 been discovered, principally from the researches of Dr. 

 Sharpey, that in the Gasteropod and Acephalous mollusca, 

 the surface of the respiratory organs, and of the cavity in 

 which they are contained, is clothed with minute cilia, wdiich, 

 by their regulated motions, impel the water along the surface 

 in a determinate direction ; and by this means a constant 

 current is kept up, and the blood exposed to the influence of 

 successive portions of the surrounding element. Similar 

 cilia clothe many parts of the external surface, and of the 

 inner surface of the alimentary canal ; and similar aqueous 

 currents are, by this means, made to flow over them ; as if 

 nature meant to avail herself of every exposed part to plant 

 there an auxiliary to the gills, so that under no circum- 

 stances should the important function of ventilating the blood 

 and the other fluids be suspended or weakened. You may 

 readily observe these interesting and beautiful phenomena f 



" Cuvicr, Mem. Mollusc, xx. 17 : Coldstream in Ediiib. New Pliil. Jouin. 

 July, 1830, i>. 240. 



t Leeuwcuhoek, who liad witnessed them in the common Mussel, witli- 



