14 STRUCTURE OF THE CRINOIDE^. 



not anal plates, I think another name preferable. The azygos inter- 

 radials always mark the anterior side of the animal, or that side 

 towards which the mouth is most approximated. The posterior side 

 is directly opposite, and indicated by the azygos ray. There are 

 thus in every Crinoid two pairs of ra3^s, the right and the left, and 

 an odd or azygos ray. There are also two pairs of inter-radii, the 

 right and left pairs, and an odd one, which is the azygos or anterior 

 inter-radius. When a Crinoid is placed with its anterior side 

 towards the observer, the left anterior ray is opposite his right 

 hand and the right anterior ray opposite his left. Such genera as 

 Gli/ptocrinus and Rhodocrinus have both regular and azygos inter- 

 radials, but PalcBocrinus, Carabocrinus and others of a similar struc- 

 ture, have only the latter. 



VII. The Mouthy the Ambulacral Grooves, and Amhulacral Orifices. 



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 Caryocriyms ornatus (Say), it is closed by a 

 valvular apparatus consisting of five or six small triangular plates. 

 In Pentacrinus caput-Medusa there is a central orifice, and, proceed- 

 ing 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 pinnulae. The grooves 

 throughout their whole length are covered over w^ith 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 fluid which serves to 

 extend or retract a set of small sucking feet which are visible on the 

 outside, one over each orifice. The margins of the grooves are 

 bordered by small erect moveable plates, which extend along the 



