ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 187 



mouth over the ventral perisome of the disc and arms. 

 These grooves are altogether outside and above the radial 

 trunks of the water- vascular, blood- vascular, and generative 

 systems, which are covered in by the ventral perisome bearing 

 the grooves. Billings, however, supposed that '^ the grooves 

 of the arms are occupied " by these tubes, and spoke of 

 them as continued into the interior of the vault by notches 

 in the first radial plates. Here he confounded the supra- 

 tegminal ambulacra with what I have elsewhere described 

 as the " ventral radial furrow,''' occupying the middle line of 

 the skeleton. All the soft parts of the arm are situated 

 above, this skeletal groove, but beneath the ambulacral 

 groove, which Miiller was accustomed to call the " Teri" 

 takelrinney'' in distinction to the skeletal one which he 

 termed the Armrinne " worin Weichtheile gelegen sind."^ 

 Billings, however, confounded the two, and because the vas- 

 cular and generative tubes which lie above the armgroove 

 (being partially contained in it) do not communicate with 

 the stomach, he supposed it to have been impossible for 

 food particles to gain access to the interior of the animal 

 from the arms of a Palteocrinoid, which, as far as we know, 

 resembled in these points the arms of a Comatula or Penta- 

 crmiis. Here he quite overlooked the fact that the ventral 

 perisome covering the arm of a recent Crinoid bears the true 

 ambulacral groove, along which, and therefore above the 

 vascular trunks, the food particles travel towards the mouth. 

 The remains of these brachial ambulacra are found in the 

 Palaocririoidea, and they undoubtedly entered the vault by 

 the ambulacral openings at the bases of the arms, which 

 Billings himself discovered, together with and above the 

 vascular trunks. Billings^ supposes that the ambulacra of 

 the Crinoids and Asterids contain the vascular, nervous, and 

 generative trunks, '' which are situated on the outside of 

 the animal, and communicate with the interior through the 

 mouth" Consequently he regarded this aperture as having 

 three functions, being (1) the oral, (2) the ovarian, and (3) 

 the ambulacral opening, and he therefore compared it with 

 respect to the last two, to the openings at the arm bases of 

 the Paloeocrinoidea. These were undoubtedly both ovarian 

 and ambulacral, as the generative organs and the water- 

 vessels passed through them to reach their respective cir- 

 cumoral centres. The ambulacra of the arms also entered 

 here and converged towards a subtegminal mouth. Billings 

 refused to admit the oral nature of this opening, which he 



' " Pentacrinus,^' loc. cit., p. 35. 

 ^ Decade iii, loc. cit., p. 19. 



