336 CONTRIBUTIONS TO THE NATURAL HISTORY 



times five, and at times even six branches to the main trunk. I should add, that, when 

 seen from above or from below, unless the body is somewhat inclined, the vertical tubes 

 altogether escape attentiou, and that the best position to ascertain their relative con- 

 nection is a somewhat oblique external side view, as in Fig. o. In Fig. 1, which rep- 

 lesents the whole system in the same position, the view of the horizontal main trunk 

 and its branches is somewhat confused, from the circumstance that it is projected upon 

 the vertical central cavity, and the prolongation of that cavity upwards and downwards ; 

 but in Fig. 5 we have only the peripheric branches arising from the main trunk, that 

 is to say, the portion seen to the right of Fig. 2, 3, and 4 ; while in Fig. 1 we have, 

 besides that half, the central axis also, as likewise in Fig. 2. 



I have described these peripheric branches as horizontal, and so they appear when 

 seen from above or from below ; but in a vertical position they are seen to be some- 

 what deviating from the same horizontal plane, the main branch reaching the perij)hery 

 somewhat higher than the secondary branch, and the vertical branches inclining slightly 

 outwards. These different branches have by no means the same functions, and are 

 not connected with the same apparatus, the vertical branches extending into the peduncle 

 or cavity from which the tentacles are protruded, while the horizontal branches com- 

 municate with vertical tubes, which follow the inner surface of the vertical rows of loco- 

 motive fringes for their whole extent. 



As there are on each side four such horizontal branches and four vertical rows of 

 fringed combs, there are also, in the whole, eight vertical superficial, chyraiferous tubes, 

 widest in the middle, and tapering upwards and downwards, which are in most strict 

 communication with the central cavity through the four horizontal tubes, and the two 

 main trunks, from which they themselves arise. The upper ends of the superficial ver- 

 tical tubes, which I may call the ambulacral tubes, terminate apparently in a blind point ; 

 at least, I have been unable to trace a direct communication between any of them and 

 the vertical tubes which follow the sides of the digestive cavity, though such a com- 

 munication is seen in the genus Bolina, as I shall mention hereafter ; it may, therefore, 

 have escaped my attention in this genus. But whether there be such communication or 

 not, the fluid circulated upwards through these tubes can be distinctly seen to retrace 

 its way downwards ; so that, in the ascending branch of the ambulacral 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, though the lower end 

 tapers gradually into very slender tubes, which extend as far as the anal area, and 

 unite there again with the central cavity. But this termination of the central tubes 

 being too narrow to allow all the liquid injected into tlie larger stem to pass through, the 



