20 BULLETIN OF THE 



alimentary tract, in the later stage the coecum has already begun to form, 

 as iu Phylactolaemata, by an outpocketing of nearly the whole of the 

 lower wall of the stomach. (Compare also Plate 1. Figs. 7, 8, and 9.) 



Very soon after the establishment of the alimentary tract, and between 

 the stages shown in Figures 24 and 25 in sagittal section, there begin 

 to appear organs which have a very considerable phylogenetic signifi- 

 cance; namely, the lophophoric ridges, ring canal, and tentacles. 



The lophophoric ridge is a fold which surrounds the mouth, and from 

 which at intervals tentacles arise. The ridge', however, arises before the 

 tentacles. The general position of the ridge, as well as its method of ori- 

 gin, may be learned from an inspection of a series of sections of the age 

 of those shown in Figures 31-34. In a section lying near the oral 

 end of the bud (Fig. 33), one finds two spaces, — a lower, which is that 

 of the stomach, and an upper, the oesophagus and atrium. This upper 

 space is broader above than below, and the cell layer which lines it is' 

 thick below, but above, or nearer to the body wall of the budding 

 individual, it is thinner. The transition from one condition to the 

 other is quite abrupt, and is marked by a salient curve (loph.). In a 

 section near the anal end of the bud (Fig. 31), it will be seen that here 

 too the inner layer is thick below and thin above. The characters men- 

 tioned are still more strikingly shown in the median section, Figure 32. 

 That the diff'erences in thickness of difierent parts of the inner layer are 

 recently acquired modifications of an earlier simpler condition is indi- 

 cated by comparing Figure 32 with Figure 30, which is from a younger 

 bud. The series of points (loph.) of transition from thick to thin epi- 

 thelium forms on the reconstructed polypide a curved line, convex above. 

 This line is the ridge of the young lophophore (compare Fig. 25, loph.). 

 I have said that the lophophoric ridge arises before the tentacles. The 

 evidence for this assertion is found in a series like that referred to above, 

 where, although the ridge exists along the entii'e side of the atrium, 

 one finds nascent tentacles in the middle region only (Figure 32, left 

 hand). 



As Figure 25, of a later stage than Figures 31-33, shows, the lopho- 

 phore curves downwards rapidly at the anal end, so that it here lies at 

 right angles to the axis of the rectum, but does not extend at all beyond' 

 the anus. Orally, there is in the median plane only the slightest trace 

 of the lophophoric ridge. By the formation of this ridge in the wall on 

 each side of the atrial chamber, the original atrio-pharyngeal cavity has 

 become separated into two regions. The space lying within or below the 

 ridge forms the pharynx and the intertentacular space ; that lying with- 



