Chap. III. GENUS PLEUROBRACHIA. 225 



as to form vertical series. But the cilia or fringes are fiir larger than any 

 vibratile cilia ever described, and their motion shows distinctly that they are under 

 the voluntary control of the animal; for their movements are neither incessant nor 

 constantly equal. They are at times accelerated or retarded, or entirely stopped, and 

 resumed at shorter or longer intervals; so that the evidence of their voluntary 

 movement is as full as can be, and, indeed, the structure which determines the 

 movements is the same as in all cases of voluntary motion. 



Fully to understand the character of the vertical rows of locomotive flappers, it 

 should be borne in mind that they are connected for their whole length with 

 vascular tubes following the same course, and which arise from the central 

 chpniferous cavity. This intimate connection leads naturally to the supposition, 

 that, besides their functions as locomotive organs, the vertical rows of flappers are 

 in some way connected with respiratory functions, and that there is between these 

 two systems the same natural physiological connection which exists in Echinoderms 

 between the inner branchiiB and the ambidacral tubes, or in Worms between the 

 respiratory vesicles and the locomotive bristles. 



The circulation of fluids, and the respiratory movements connected with this 

 circulation, are, almost tbroughout the animal kingdom, in direct relation to loco- 

 motion, even in the higher animals. Among Polypi, the dilatations and con- 

 tractions of the body renew constantly the water which fills their cavity, and 

 provide them with a fresh supply of aerated water. The same is the case among 

 Medusae. For, even where there is no distinct, individualized system of respiratory 

 organs, it is obvious that a constant renewal of the surrounding medium, by means 

 of which oxygenation takes place, is an essential condition for the maintenance of 

 life; and where there are no special organs adapted to this purpose, the main 

 movements of the body supply the deficiency. The water-pores in Echinoderms, 

 through which their main cavity is constantly filled with fresh sea-water, undoubtedly 

 perform a similar office. Again, among Mollusca, respiration and locomotion are 

 still more intimately connected ; but in a manner which differs decidedly from what 

 we observe in higher animals. For here, by the ddatation and contraction of the 

 respiratory cavities, and the circulation of the blood through the respiratory organs, 

 the body is amply supplied. But, unless Acephala open their valves, unless they 

 expand and contract alternately the Avhole body, the supply of fresh aerated water 

 must be much less ; and I doubt whether oysters and clams could be kept alive if 

 their valves were shut constantly by pressure, and muscular motion, the contraction 

 and expansion of the large bundles which preside chiefly over locomotion, were 

 prevented from coming into play in aid of the vibratory cilia of the mantle and 

 gills. The manner in which the respiratory cavity is shut in so many Gasteropods, 

 unless the fleshy parts are fully expanded, shows plainly that here again there 

 VOL. III. 29 



