46 DISCOPHORiE. Part III. 



extends from one oculiferous lobe (/) to the other, in the form of a segment of 

 a circle, being broadest at the middle, and narrowing each way till it passes into 

 the disk at the ends. Its base (PI. XP. Fig. 17 P ; PI. XF. Fh/. 4 tt) is nearly 

 on the same line with the bases of the tentacles, and also corresjjonds to the 

 curved edge of the disk. The corners of the trumj)et-shaped proboscis have become 

 prolonged to a great extent (PI. XI"". Fig. 17 a^), so that they reach half way to 

 the margin of the disk, running out into a point, and have a strong likeness to 

 those of the adult (PI. VI. Fig. 1), as far as their general outline is concerned. 

 The edge of the lips is either wavy, lobed, or fringed all around. The mouth 

 (PI. XP. Fig. 17 cc'), or cavity of the proboscis, is also very much like that of the 

 adult, not only by its four-sided form, ]3ut by its furrow-like prolongations into each 

 of the four elongate corners (a^). Tlie digestive cavity [b) is comparatively smaller 

 than in the last jjhase, whilst, by the increasing diameter of the disk, the radiating 

 canals [c e) have elongated considerably. The eight simple radiating canals (e) are 

 now narrow tubes, which stretch in direct lines from the digestive cavity to the 

 middle of each marginal canal [e^ mc). The eight forked canals {c) are even 

 narrower than the simple ones, and are either twice or thrice forked on each side. 

 The forks (e^ (?), as in the adult, all lead to the margin between the oculiferous 

 lobes. The new forks (c^ c^) arise from the marginal canal {mc), and channel their 

 way toward the centre of the dislv until they meet with the main canal, at about 

 one third of its length from its entrance (c^) and near where all the other forks 

 meet. The marginal canal {mc) is as yet quite broad, at least opposite the entrance 

 of the simple radiating canals (e), but becomes narrower as it extends right and 

 left of this point. 



In order that the structure of these canals may be fully understood, we refer 

 to a figure (PI. XI^ Fig. 13) representing an actual transverse section of one of 

 the simple canals {e, and Fig. 17 e), and two of the branches of the forked canals 

 on each side {Fig. 13 c). By this it may be seen that the canals are not inclosed 

 hy one and the same wall ; but that the upper or roof-like side {d f) is covered by 

 the inner wall {i^) of the upper floor of the disk, and that the lower side is 

 inclosed by the inner wall {i") of the lower floor of the disk. Here, too, we may 

 see that these two inner canal-bearing walls («'* i°) are suspended or svii^ported by 

 a cellular network, which fills all the space between them and the outer walls 

 {i^ ^'), and also that the ridge {d f) of the canals, as well as the lower wall, 

 is connected with the outer walls of the disk by thicker meshes, or groups of 

 cells with filamentary prolongations {a (i). The broad, concentrically plicate band 

 {Fig. 17 m m^), Avhicli first made its appearance in the fourth stage previous to this 

 {Fig. 4 m m^), occupies nearly one half of the diameter of the disk from the 

 margin inwards. It does not, however, seem to have grown more plicate, but, on 



