Chap. II. STRUCTURE OF THE ADULT. 57 



digestive cavity greatly, and finally reducing it to a narrow space, between the 

 base or pillars of the oral appendages (same figure, b) and tke central projection 

 of the upper floor, o. In very old specimens, when the spawning season has^ 

 passed and the ovaries and spermaries have discharged their contents, the central 

 projection of the upper floor has become so prominent as to assume the form of 

 a four-sided pyramid, filling the whole space between the four arms, and termi- 

 nating as a four-sided roof, the point of which hangs down towards the external 

 oral aperture, and, in the end, the contact between the arms and this plug is so 

 close, that probably all connection between the surrounding medium and the main 

 cavity is stopped, except along the angles of the mouth and the emargination of 

 its sides leading to the genital pouches. The roof-like termination of the plug 

 presents, at this time, as regular facets, as a four-sided pyramid with truncated angles. 

 The development of the genital apparatus, as it progresses, is accompanied by 

 equally great changes in the form of the surrounding parts and their relation to 

 one another. At first we notice only the oval depi'essions on the lower surface 

 of the loAver floor, in the interambulacral sj^''^^^'^ near the intervals between two 

 projecting angles of the oral tube, on the outer side of which arise the digitate 

 bodies ; but in proportion as these depressions deepen, and the corresponding parts 

 of the main cavity above them encroach upon the bases of the radiating tubes, to 

 form distinct genital pouches, the lower surface of the gelatinous disk, corresponding 

 to the interval between two genital pouches, projects in the shape of a keel between 

 them (PL IX. FIr/s. 0, 7, 8, and 9 d, and Fig. 5 o iai the distance), thus tending 

 to isolate more and more the genital pouches from the digestive cavity, until the 

 central prominence of the gelatinous disk has been entirely developed, when they 

 are fully separated as distinct cavities, preserving only a narrow communication 

 with that cavity, through the channels marked s in Figs. 5, 8, and 9. At the same 

 time the lower floor has become greatly thickened, at points marked e in Figs. 

 7, 8, and 9, in consequence of which the sexual pouches are imderlaid by ample 

 cavities communicating freely with the surrounding medium, from which they are 

 separated, however, by thin floors stretching .across the whole lower side, and sup- 

 ported by two stronger arches, which, seen from above, as in PI. VII. Fig. 1, appear 

 like folds arising from the inner angle of each pouch and diverging towards its outer 

 angles. These arches (PI. IX. Fig. 6 p) are distinctly seen in a transverse section 

 of a genital pouch, where the floor of the .cavity is marked p ; they are equally 

 well seen in an oblique side view of the genital pouch {Fig. 9 p), and in a longi- 

 tudinal section through a pouch {Fig. 8 p). It thus appears that while the genital 

 pouches {Figs. 5, 6, 7, 8, and 9 n) communicate freely with the main cavity through 

 the channels (s), they have no direct communication whatever with the wide cavities 

 (/), which are immediately below them ; though these cavities, with their round 



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