232 CTENOPIIORiE. Part II. 



actiual ends of the yiiperlicial vertical tubes, -whicli I may call the ambiilacral tubes, 

 terniiuate as blind canals ; at least, I have been unable to trace a direct com- 

 munication between any of them and the vertical tubes which follow the sides of 

 the digestive cavity, though such a communication is seen in the genus Bolina, 

 as I shall mention hereafter. The tluid circulated upward through these tubes can 

 be distinctly seen to retrace its way downward ; so that, in the ascending branch 

 of the aml)ulacral tubes, the fluid injected through its horizontal branch is moved 

 up and down alternately. This is also the case with the lower branch of the 

 same vertical tubes, and, though the abactiual end tapers more gradually, it 

 terminates also in blind canals ; and I was mistaken in formerly supposing them 

 to open again into the main cavity. The movement in reality takes place in 

 the following manner. Each of the eight horizontal tubes fills its vertical ambu- 

 lacral branches, the fluid flowing, at the junction of the vertical tube with the 

 horizontal stem, in opposite directions, upward and downward ; then, flowing back 

 through the same channels during the contraction of the mass of one side of the 

 body, it is pressed into the horizontal tube, and returns to the centre of the move- 

 ment to pass into the opposite side of the body. There can be no doubt, that 

 the liquid moves decidedly to and fro in the ambulacral tubes and returns to the 

 central cavity through the horizontal tubes, and that the dilatation of the four tubes 

 of one side alternates with the dilatation of the four tubes of the opposite side ; 

 but in each vertical ambulacral tube the motion of the fluid is an undulatory one, 

 owing to the alternate dilatation and contraction of the tube itself 



The movement of the fluid in these tubes can be traced very satisfactorily 

 when following the course of the minute granules of colored matter suspended in 

 the water after injection ; but even in fresh uninjected specimens, the circulation 

 can be tolerably Avell traced by watching the small particles of undigested food 

 suspended in the mixture of water and chyme which is circulated throughout this 

 system. As in Polypi, the whole mass of digested food, comminuted and reduced 

 to a very uniform state, but in which the parts cajoable of ]:)eing assimilated are 

 still mixed with parts of the refuse matter, is emptied bodily into the ch3'miferous 

 cavity, and, with a certain c^uantity of w^ater introduced in the same w'ay into 

 this cavity through the mouth, kept in a constant, regular, undulatory circulation 

 throughout life. But as there is a double outlet tbrough which this system can 

 discharge its contents on the side of the circumscribed area, the circulation is more 

 or less active, all the tubes more or less turgescent, and the whole cavity more 

 or less dilated, as the quantity of fluid in circulation is greater or less ; which, to 

 some degree, changes the relative position of the tubes and of the central cavity. 

 When very full, the wider central space is consideral^ly raised ; while in a state 

 of relaxation it sinks lower down, nearer the abactinal extremity of the body. As 



