Chap. III. GENUS PLEUROBRACIIIA. 233 



long as the circulatory sA^stem is relaxed, the ambulacral tubes are very much 

 contracted, and their diameter is much less than under other circumstances, and 

 by no means equals the width of the vertical rows of locomotive flajapers ; but 

 when turgescent and full, they swell beyond their width. The force which acts 

 in propelling the liquid through the system is not the same throughout. The 

 alternate contractions of the two sides result from the regularly alternating muscular 

 contractions of the two sides of the body ; but the main cavity in its central part 

 is entirely lined with vibratory cilia, so that even when the body is perfectly at 

 rest, the fluid is maintained in a constant rotatory motion through their agency. 

 I have repeatedly and distinctly seen these cilia playing round the abactinal opening 

 of the digestive cavity, and upon the walls of the central chymiferous cavity, as 

 well as upon the walls of its main horizontal stems, upon the walls of the coeliac 

 tubes, and upon the walls of the two forks of the funnel. I have been unable, 

 however, to discover similar cilia within the secondary horizontal tubes and the 

 vertical ambulacral tubes ; though recently I have noticed them in the vertical 

 tubes of the tentacular apparatus, where I had failed to discover them before. 

 However, the contractions of the spherosome are so powerful that the vibratory 

 cilia can do but little, of themselves, to keep the fluid in motion in some of 

 these tubes. I should also add, that even the walls of the central chymiferous 

 cavity, Avhere they are most distinctly lined with vil)ratory cilia, are nevertheless 

 distinctly contractile; and that the capacity of the cavity is not only increased and 

 reduced in a passive manner by the accumulation of fluid or its expulsion, but 

 also actively by the contraction and dilatation of the walls themselves. How the 

 contents of this circular system are diffused into the suljstance of the body for 

 nourishment is not very plain, as there are no capillaries, jjut everywhere broad 

 tubes. From the cellular structure of the whole mass, howevei', we may infer 

 that assimilation takes place by a process of endosmosis and exosmosis. If this 

 vicAV is correct, we should consider the two coeliac tubes upon the middle of the 

 main walls of the digestive cavity as the nourishing vessels of the stomach ; the two 

 horizontal trunks as two respiratory vessels, branching into eight branchial vessels, 

 which are the main trunks of the eight ambulacral vessels; and the vertical funnel 

 as a vascular cloaca, discharging its contents through two distinct apertures, the 

 coeliac apertures, on the sides of the circumscribed area near the abactinal centre. 

 The vertical tubes of the tentacular apparatus seem to have a pecidiar function, 

 and to be directly connected with the movements of the tentacles, and these move- 

 ments again to be connected Avith the alternate contraction of the two halves of 

 the bod}^, as there are no parts Avhich undergo so extensive changes in their size, 

 and in their state of contraction and dilatation, as these sacs. But their structure 

 is so comjolicated as to require a minute description. The two tentacles with their 

 VOL. III. .30 



