226 Geological Survey of Canada. 



VII. The Mouth T Ambulacra! Grooves, and Ambulacra! Orifices 



u The space on the upper part of the body surrounded by the arms 

 is called the ventral surface, and, by some authors, the vault. It 

 is covered with plates, which are usually smaller than those of the 

 walls of the cup, and disposed without any observable order. 

 The mouth is a circular or oval aperture, situated either in the 

 centre of the vault or between the centre and the margin of the 

 cup, towards the anterior side or below the margin in the side. 

 It sometimes consists of a tube called the proboscis," which rises 

 from two or three lines to more than an inch above the surface. 

 In some species, such as Caryocrinus ornatus (Say), it is- closed by 

 a valvular apparatus consisting of five or six small triangular 

 plates. In Pentacrinus caput-MedusGs there is a central orifice, 

 and, proceeding from it, five ambulacral grooves on the surface 

 of the vault, which radiate outwards and divide into ten before 

 reaching the margin. The ten grooves proceed straight, to the 

 bases of the ten secondary rays or free arms, and are continued 

 upon them to their extremities. The main grooves send out 

 branches to all the divisions of the arms and to each of the pin- 

 nulse. The grooves throughout their whole length are covered 

 over with a soft skin, through which there are numerous minute 

 circular perforations arranged in two rows, one along each side 

 of the groove. These orifices are supposed to be passages for the 

 Huid which serves to extend or retract a set of small sueking feet 

 which are visible on the outside,, one over each orifice. The mar- 

 gins of the grooves are bordered by small erect moveable plates y 

 which extend along the sides like a fence of minute palings* 

 These are the marginal plates of the ambulacral grooves." 



"The grooves are covered passages, along which are conveyed 

 from the interior of the body to the arms and pinnulse a number 

 of tubular vessels whose functions appear to be of great importance 

 in the physiology of the Crinoids. As the eggs from which the 

 young are produced are developed in the pinnulse, no doubt there 

 must be an organ of some kind connected with their generation 

 which communicates with the viscera of the animal by passing 

 along the grooves. Another set of vessels are the aquiferous 

 canals, consisting of long, slender tubes for the conveyance of the 

 fluid by which the sucking feet of the arms and pinnulse are ex- 

 tended or retracted. To these must be added the blood-vessels,, 

 nervous filaments, and muscles. Traces only of these have been 

 actually observed, but the almost perfect identity in structure be- 



