226 Description of the Plates. 



b 2. Arborescent eoecum of radius III, displaced, as is its fellow, 

 into the next interradial space. 



b 3 and b 4. Terminations of coeca of radius V attached to the 

 dorsal integumentary skeleton by a mesentery. 



c. Stomach proper, bulging into the cavity of the several rays 

 at a lower level than the coeca, b, but only for a short dis- 

 tance. Ligaments may be observed passing up on either 

 side of c from the ambulacral ossicles, by means of which 

 the stomach can be retracted after being protruded, as it 

 often is by the animal when feeding. To the right of c is 

 seen one of the interradial septa to which the ducts of the 

 generative coeca on either side are attached. 



d 1 . Ampullae of ambulacral feet of radius IV ; they are biserial 

 on either side, in correspondence with* the four sucker-like 

 feet which communicate with them through conjugate fora- 

 mina formed by the alternating apposition of emarginations 

 of the ambulacral ossicles ; for which see fig. 3, pi. i., 

 Wright, British Fossil Echinodermata from the Oolite, 1862 ; 

 Gaudry, Ann. Sci. Nat., Ser. iii., torn, xvi., pi. 13, fig. i. 



d 2. Ampullae of radius V. 



e. Subcentrally-placed anus. 



y. Origin of ext-ensor muscle of radius I from inner surface of 

 centre of dorsal integument. It is by the action of this 

 muscle that the distal extremity of the rays and the com- 

 pound eyes they carry, have their ordinary up-turned direc- 

 tion, as shown in this figure, given to them. 

 J" I . Distal termination of extensor muscle of radius V. 

 g. Madreporic canal and plate displaced backwards into the inter- 

 radial space of the bivium, opposite to which it is placed in 

 the natural position of the parts. The madreporic plate 

 being porous, and the madreporic tube, in spite of a some- 

 what complicated internal structui'e, being very readily 

 permeable by fluid, it is easy to see how the sea-water can 

 find its way into the water-vascular ambulacral system with 

 which the madreporic canal is connected by its junction to 

 circum-oral water-vascular ring. The ' heart ^ of the pseud- 

 haemal system is inclosed in the same membranous sheath 

 with the madreporic canal, and, like it, communicates with 

 a circum-oral annular vessel, which lies inferiorly to the 



