vin ECHINODERMATA WATER VASCULAR SYSTEM 431 



as often as do the arms and their food grooves. Their course is more 

 or less markedly zigzag, and they give oft* at the angles thus formed 

 (i.e. alternately) lateral tentacle canals. Each of these latter runs to 

 a group of three small tentacles at the edge of the food groove, and 

 here divides into three canals, which enter the three tentacles and 

 form their cavities. 



Tentacle canals are wanting in all cases where food grooves are 

 wanting, which is the case in Actinometm over a great part of the 

 arms, and in some species of Antedon in certain proximal pinnules of 

 the arms. 



All authors agree in maintaining that the inner epithelium of the water vascular 

 system in the Crinoids differs from that in all other Echinoderms in not being 

 ciliated. A band of longitudinal muscle fibres runs in the wall of the canals along 

 the side turned to the food groove. The lumen of the canals is at certain points (i.e. 

 where the tentacle canals branch, or at the commencement of these canals) traversed 

 by muscular fibres. This arrangement perhaps fulfils the function of the valves 

 found in other Echinoderms. 



D. The Ambulaeral Appendages. 

 (Tube-feet, Tentacles, Feelers, Ambulacral Papillse, etc.) 



1. Holothurioidea. The following facts require first of all to be 

 emphasised. 



a. In all Holothurioidea, a smaller or greater number of ambulacral 

 appendages (10-30) are developed as tentacles near the mouth. 



b. The Synaptidce and Molpadiidce have no ambulacral appendages 

 except these tentacles. 



f. In all other Holothurioidea besides the tentacles there are 

 tube-feet (and papillae) varying greatly in number (often very 

 numerous), in structure, and in arrangement. 



d. These tube-feet (and papillae) are found either only on the 

 radii, one or two or more longitudinal rows being arranged in each 

 radius, or else they are distributed, usually in an irregular manner, 

 over some or all of the interradii. The arrangement of the tube-feet 

 is not of great systematic importance, since even within one and the 

 same genus (e.g. Cucumaria), all the intermediate stages between a 

 strictly radial and an altogether scattered arrangement can be 

 observed. 



e. Where the ventral and the dorsal surfaces are distinctly differ- 

 entiated, the ambulacral appendages are developed on the ventral side 

 (in the trivium), normally as loeomotory tube-feet with sucking discs 

 supported by perforated plates : on the dorsal side, on the other 

 hand, they take the form of conical non-locomotory papillae, which 

 have either a rudimentary perforated plate at the narrow tips or none 

 at all. 



