206 HYDROID^. Part IV. 



centric layers. When the thickness of the sheath is examined in profile, it appears 

 to be fibrous (PI. XX. Fi<j. 2) ; but this is owing to the minute concentric layers 

 of which the tube is composed; since, in a view from the superficies (PI. XX. 

 Fig. 1, a), nothing like fibres is to be seen. In the young hydroid (PI. XX. Firf. 6) 

 the outer (a) and inner {b) walls are built up of columnar cells of much smaller 

 size than in the adult. See p. 203. 



Proles medusoidea. No definite information has been obtained about the cellular 

 structure of the embrj'o medusa-bud, in its earliest stages. At that time the outer 

 and inner walls appear like perfectly homogeneous and very transparent layers. 

 As soon, however, as the innermost wall (PI. XVIII. Fig. 9, P^ c) has been established, 

 the outer wall {h) may be seen, in profile, to be composed of wedge-shaped, faintly 

 granulated, prismatic cells, the broader ends (PI. XIX. Fig. 7% a) of which are turned 

 outward, while the narrower ends {b) form the inner surface of the wall. The 

 inner ends {Fig. 7''), in a front view, present an irregularly polygonal mesh. At 

 a still later period, when the tentacles are considerably elongated (PI. XVIII. 

 Fig. 13, a), but before they coil inwards into the cavity of the disk, the cells of 

 the outer wall (PI. XIX. Fig. 7, a) have become proportionally broader and shorter, 

 in fact, nearly as broad as long, and have very little of the wedge-shaped form 

 of earlier stages. The outer wall {Fig. 7, a}) of the tentacles is composed of the 

 same sort of cells as are found in the outer wall of the disk. The cells of the 

 middle wall {Fig. 7, b) are very obscure, except in its prolongation in the inner 

 wall (i^) of the tentacles. There they have much the same character as those 

 of the outer wall {a}). 



About the time the medusa is ready to drop from the parent stem, or just 

 at the time when it becomes free, the cells of the outer wall (PI. XIX. Figs. 14, a, 

 and 14") have expanded laterally, so as to be a great deal broader than long. 

 When seen in front {Fig. 14*) they are conspicuous for their irregular form, giving 

 the disk the appearance of being covered by a network of irregular meshes. In 

 profile, the inner ends appear like slightly prominent papillse {Fig. 14, a a^). Each 

 cell contains a very large, distinctly granulated mesoblast Fig. 14", b), and each 

 mesoblast a very faint entoblast (e). The cells of the outer wall (PI. XX. Fig. 8, «') 

 of the short papilliform tentacles of a medusoid, which lays its eggs before becoming 

 free (PI. XVII. Fig. 15), are broadly pear-shaped, and very transparent, resembling 

 very closely the cells of the outer wall in a much younger stage (PI. XIX. Figs. 

 7" and 7''). The only trace of organization that could be found in the middle 

 wall (PI. XIX. Figs. 11, b, and 14, b b^) of the disk, was a faint horizontal striation 

 • (PL XIX. Fig. 11, d), caused by rows of granules {Fig. 12, a a^) arranged in close 

 parallel lines; and even these were brought out by the agency of water or alcohol. 

 There seems to be only a single layer of these granules, judging from a profile 



