Chap. III. GENUS PLEUROBRxVCIIIA. 231 



base ; so that it may with equal propriety be said, that, on the whole, there are only 

 four tubes, the two horizontal ones branching soon again into two. In the dilated 

 state of the main trunk, but when the In-anches arising from it are in a state of 

 contraction, these all seem to originate from one common cavity, and the four 

 horizontal tul)es appear independent of each other, while the two vertical ones are 

 brought so close together as to look like one, — making altogether five branches. 

 In another state of contraction, the two vertical tubes may seem imited, and the 

 two pairs of horizontal ones also, when there appear to be only three branches 

 to the main trunk. Unless the dilatations and contractions of these curious rami- 

 fications of the stems have been watched for a long time, these diflerences may 

 remain unnoticed ; but when fully understood, there is no contradiction in the 

 apparently conflicting statements, that there seem at times to be three, at times 

 four, at times five, and at times six branches, to the main trunk. I should add, 

 that when seen from the actinal or from the abactinal pole, unless the bod}^ is 

 somewhat inclined, the vertical tubes altogether escape attention, and that the best 

 position to ascertain their relative connection is an external side view, as in Fig. 

 15. In Fig. 22, which represents the whole system in the same position as Fig. 15, 

 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 actinal 

 prolongation of that cavity v;pward and downward ; but in Fig. 15 we have only 

 the peripheric branches arising from the main trunk, that is to say, the jaortion 

 seen to the left in Fig. 23, while in Fig. 22 we have, besides that half, the 

 central axis also, as likewise in Fig. 23. 



I have described these peripheric branches as horizontal, — and so they aj^pear 

 when seen from above or from below ; but in a vertical jjosition they are seen 

 to be somewhat deviating from a horizontal plane, the anterior and posterior 

 branches reaching the periphery at a greater distance from the abactinal pole than 

 the lateral branches, and the vertical branches inchning slightly outward. These 

 different branches have by no means the same functions, and are not connected 

 with the same apparatus; the vertical branches, which I have called interambulacral 

 or tentacular tubes, extending to the disk from which the tentacles are protruded, 

 while the horizontal branches communicate with vertical tubes, — the ambulacral 

 tubes, — which follow the inner surface of the vertical rows of locomotive flappers 

 for their whole extent. 



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

 of locomotive flappers, there are also, in the whole, eight vertical superficial chy- 

 miferous tubes, widest in the middle, and tapering upward and downward, which 

 are in direct communication with the central chymiferous cavity through the four 

 horizontal tubes and the two main trunks, from which they themselves arise. The 



