﻿OK THE ACALEPH.E OF NORTH AMERICA. 337 



liquid moves here also up and down. The movement, in reality, takes place in the 

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

 branches, the fluid flowing, at the junction of the horizontal tube with the vertical stem, 

 in two opposite directions, upwards and downwards. A small quantity passes through 

 the narrow prolongation of the tubes below back into the main cavity ; but the greater 

 portion flows back during the contraction of the mass which has been moved upwards, 

 is pressed into the horizontal tube, and returns to the centre of the movement to pass 

 into the opposite side of the body. It may be, also, as mentioned above, that a small 

 portion of the fluid passes through exceedingly minute tubes into the vertical tube of 

 the stomach, and back into the central cavity, in the same manner as upon the anal 

 extremity, as this is really the case in Bolina. However, this communication above and 

 below is too narrow to establish a direct onward circulation ; the liquid moves decidedly 

 to and fro in the ambulacral tubes, and returns chiefly to the central cavity through the 

 horizontal tube, and, what is still more interesting, the dilatation of the four tubes of one 

 side alternates with the dilatation of the four tubes of the opposite side. Moreover, 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, as shown in Plate II. Fig. 6. 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 injec- 

 tion ; but even in fresh specimens uninjected, the circulation can be tolerably well 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 capable of being assimilated are still mixed with the refuse matter, is indiscrim- 

 inately emptied into the main cavity of the body, and, with a certain quanity of water 

 introduced in the same way into this cavity through the mouth, kept in a constant regular 

 undulatory circulation throughout life. But as there is a double outlet through which this 

 system can discharge its contents on the side of the anal 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 the central cavity. When very full, the wider 

 central space is considerably raised, while in a state of relaxation it sinks lower down, 

 nearer the anal extremity of the body. As long as the circulatory system is relaxed, 

 the ambulacral tubes are very much contracted, their diameter is much less than under 

 other circumstances, and by no means equals the width of the vertical rows of loco- 

 motive fringes ; but when turgescent and full, they swell beyond their width. The 



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