Chap. III. 



GENUS IDYIA. 



281 



greatly vary when it is more or less distended [Figs. 3 and 4), but when it is 

 entirely empty and the interamlndacra have subsided, its walls are pressed against 

 each other from the side. The whole surface of the digestive cavity is lined with 

 a very j)eculiar epithelium (PI. 11. Fig. 19), resembling somewhat a vibratile epi- 

 thelium, the cilia, however, being much stouter and Ijlunter than those of ordinary 

 vibratile cells, and resembling somewhat the baculi of the retina. Between them 

 there ai-e rows of branching pigment cells. 



The structure of the mouth still requires further investigation. All my efforts 

 to make out the microscopic structure of its edge have thus far been unavailing. 

 From figures drawn in a natural size (PI. I. Figs. 2" and 4, and PI. II. Fig. 10), it 

 may be seen that the stout vibratile fringes lining the digestive cavity, and the 

 pigment cells intervening between them, are arranged near the edge of the mouth 

 in vertical rows, giving it a striate appearance (PL I. Fig. 2"^, and PI. II. Fig. 19 

 magnified). When the mouth gapes, the abrupt termination of these parts gives 

 it a well-defined outline, winch may be waving as in PI. I. Fig. 2'', or double S 

 shaped as in Fig. 9, or straight by the apposition of the two sides when the mouth 

 is closed, as in Fig. 4. Outside of this well-marked edge and between it and the 

 circular oral tube {Figs. 4 and 2") is a pale circle, the most movable and most 

 powerfully contractile part of the whole body. Fig. 19 of PL II. represents that 

 band magnified, in connection Avith the rows of viln-atile cilia and pigment cells of 

 the digestive cavity on the right of the figure, and the superficial stellate pigment 

 cells scattered between the epithelial cells of the outer surface, on the left of the 

 figure. The baud without pigment cells, to the left of niunber 19, corresponds to 

 the pale circle surrounding the mouth. It is evident, from the glimpses I could 

 obtain of this part under the microscope, that cells arranged concentrically form 

 the edge of the mouth, and that its striated appearance is /-,-^. 97. 



owing to the fibre-like aspect of the angles of these cells, over 

 which a thick epithelium without pigment cells reaches from the 

 outside to about as near the edge itself as the pigment cells 

 extend on the inner surfiice. It is Avith this sharp edge that 

 Idyia cuts its prey. While swimming in pursuit of it with 

 the- mouth gaping, the anterior and posterior interambulacra 

 are so contracted as to appear more or less deeply emarginate, 

 and the sides assume the form of two broad lips {Figs. 1 and 8). 



The chymiferous cavity {Fig. 97) is very short, though 

 wide ; indeed, much shorter than in any other type of this 

 order, and the digestive cavity opens into it through a long 

 fissure, which may gape and contract so as to render it very 

 difficult to trace its outlines, unless the whole chymiferous system be fully distended 



VOL. III. 36 



uniiel, or central chymiferous 

 cavity of 

 Idyia roseola Ag. 

 capsule of the eye-speck. — b eye- 

 speck. — e tubercle of the eye- 

 speck. — /funnel. — ri cceliac tube 

 upon the distended digestive cav- 

 ity. — T cadiac tube, supposing the 

 digestive cavity empty. — lU* lat- 

 eral chymiferous tubes. — ^2 ^3 an- 

 terior chymiferous tubes. — d cce- 

 liac aperture. 



