MUSEUM OF COMPARATIVE ZOOLOGY. 135 



3. Development of the Tentacles. — Nitsche ('75, p. 359) observed that 

 both layers of the bud went to form the tentacle in Phylactolaemata, 

 and that the inner layer was derived from the outer layer of the polyp- 

 ide ; the outer, on the contrary, form the inner cell layer. He states, 

 moreover, as already mentioned, that the oral tentacles arise first, then 

 those of the outer row of the lophophore arms, of which the basal are 

 fully formed before the terminal ones. The tentacles of the inner row, 

 he says, are formed last, and in Alcyonella are yet lacking when the 

 polypide is first evaginated. 



My own observations confirm in general those of Nitsche. The long- 

 est tentacles in a polypide of about the age of that shown in Figure 77, 

 Plate IX., are those arising from the region of transition from the circum- 

 oral ring canal (can. crc.) to the outer lophophoric ring canal (can. crc. 1 ). 

 The tentacles lying near the median plane, and in front of the mouth, 

 are somewhat shorter than these (75 /x : 52/x). The tentacles situated 

 near the proximal extremity of the inner lophophoric ring canal (can. 

 crc") are still shorter (50 /x). Those situated at the tips of the lopho- 

 phore arms are at this stage about 30 \x in length. The tentacles behind 

 the mouth, arising from the supraganglionic part of the ring canal (can. 

 crc.'"), are shortest of all at this stage (15 jx). 



The two layers which, as we have seen, go to form the upper wall of 

 the ring canal in all its parts, are the ones which give rise to the ten- 

 tacles. In Figure 74, ta.', Plate VIII. (compare Fig. 51, ta.'), young oral 

 tentacles are cut transversely at different heights. The circu moral part 

 of the ring canal is seen at a point (can. crc.) near which it opens into 

 the cavity of the lophophore arm. The plane of the section passes ob- 

 liquely upward and anteriorly from this point. The most posterior ten- 

 tacle in the lower part of the figure is cut at the base. The calibre of 

 the canal (including its walls,) is evidently much enlarged at this point. 

 The enlargements of the canal at the base of the tentacles are seen also 

 in Figure 78, can. crc, Plate IX. The more anterior tentacles in Figure 74 

 show the two layers well marked, but as yet enclosing no lumen. Since 

 the tentacles arise from the ring canal at intervals only, the ring canal 

 is a tube (or groove) whose lumen is alternately constricted and ex- 

 panded laterally as well as vertically. The lumen is, indeed, often so 

 small between the tentacles that the ring canal appears divided into 

 separate chambers by a series of transverse septae, which, however, are 

 always penetrated by an opening (Fig. 78, can. crc). Figures 73 and 77, 

 ta.', show, in longitudinal section, successive stages in the development 

 of the oral tentacles. The formation of tentacles begins by a rapid cell 



