338 CONTRIBUTIONS TO THE NATURAL HISTORY 



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 muscular contractions of 

 the two sides of the body regularly alternating ; but the main cavity in its central 

 parts 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 lower opening of the 

 digestive cavity, and upon the walls of the vertical, central, circulatory cavity, as well 

 as upon the walls of the main horizontal stems, and upon the walls of the inferior 

 vertical funnel, even as far as its two forks which diverge below. I have been unable, 

 however, to discover similar cilia within the secondary horizontal tubes, or the vertical 

 ambulacral tubes. I have also failed to discover them in the vertical tubes of the ten- 

 tacular cavity, though they may exist there also. However, the contractions of this 

 latter cavity by muscular power are so extensive, that the agency of vibratory cilia 

 does not seem to be required to keep the fluid in motion in that part of the system. 

 I should nevertheless add, that even the walls of the central cavity, where they are 

 most distinctly lined with vibratory cilia, are also fibrous, and that these fibres are dis- 

 tinctly contractile, and 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 substance of the body for nourishment is not very plain, 

 as there are no capillaries, but everywhere broad tubes. From the structure of the whole 

 mass, however, we may infer that assimilation takes place by a process of endosmosis 

 and exosmosis. If this view is correct, we should consider the two ascending tubes 

 upon the middle walls of the digestive cavity as the nourishing vessels of the stomach ; 

 the two main 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 below as a vascular cloaca, discharging its contents through two distinct aper- 

 tures on the sides of the anal area near the lower centre. 



The vertical tubes ascending into the sacs from which the tentacles issue seem 

 to have a peculiar function, and to be directly connected with the movements of the 

 tentacles, and these movements, again, to be connected with the alternate contraction 

 of the two halves of the body, as there are no parts which undergo so extensive changes 

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

 structure is so complicated as to require a minute description. 



The two tentacles (Plate IV.), with their elongated cavity and the vertical tubes, 

 which penetrate into the base of the sacs, constitute, indeed, most complicated pieces of 



