5G DISCOPHORiE. Part III. 



main cavity, the entrance to it is a straight line between these opposite folds (PI. 

 VI. Fig. 1). The oral aperture presents thus a longitudinal fissure, and the two arms 

 which have a tendency to approximate one another on opposite sides, are respect- 

 ively on the two sides of that longitudinal fissure. There is, therefore, a sort of 

 hilaterality introduced in these radiated animals, in consequence of their peculiar 

 mode of increase of the oral appendages, and their tendency to diverge from the 

 uniform radiating disposition which they exhibited at first. The rectilinear radi- 

 ation of the oral appendages, so conspicuous in the young, is also further lessened 

 l)y the circumstance, that the enlargement of their margin causes them to wave 

 to and fro in folds which widen gradually from the tip of the arms towards their 

 base, where they are so wide as to become entirely one-sided. In this stage of 

 their development, the oral appendages have become so thick, especially at their 

 base, and the oral tube, which at first was quite distinct from the prolongations 

 of the corners of the mouth, has become so intimately connected witli the base 

 of the arms, that these parts have, in a great measure, lost their prior flexibility, 

 with the exception of the margin surrounding the outer oral aperture, and instead 

 of hanging loosely down, the arms have a tendency to remain stretched horizon- 

 tally, their tips only bending downwards; and when the gelatinous disk is strongly 

 arched, and its margin l^ent inward toward the appendages of the lower surface, 

 as in PL VIII. Fig. 1, and PI. VI. Fig. 2, the arms do not project at all beyond 

 the outlines of the body, but are, on the contrary, coiled up sideways in the con- 

 cavity formed by the arching of the whole body. 



On separating the mesial fold of the arms, and turning sideways their opj^osite 

 margins, the short canal between them, which leads to the central cavity, appears 

 still quadrangular (PI. VI. Fig. 3). But here also, great changes have taken place 

 m the outline of the sides of that opening, as a comparison with PI. XP. Fig. 17, 

 may show. The angles of the inner opening of the oral tube are more prom- 

 inent, in consequence of the closer folding of the back of the arms, and the sides 

 of the quadrangular aperture are deeply emarginate, while they are straight in the 

 young; and these emarginations lead to the channels, by which the genital pouches 

 communicate with the main cavity. The main cavity itself is at first an open 

 space between the upper floor or gelatinous disk of the umbrella, and the lower floor 

 from which arises the oral peduncle ; but in proportion as the genital pouches, 

 which at first are only small, enlarge so far as to occupy almost entirely the cen- 

 tral space where their inner margins are brought close together, as in PI. VII. 

 Fig. 1, the lower surface of the gelatinous disk begins to bulge in the centre, and 

 to press down between the inner angles of the four genital pouches, until they 

 reach the upper and inner surface of the oral appendages, with which thej^ are 

 brought into immediate contact (PI. IX. Figs. 8 and 9 r>), thus lessening the main 



