THE DEVELOPMENT OF ALCYONIUM DIGIT ATUM. 
81 
cavities lined with the cytoplasm of the scleroblast, each 
cavity shaped exactly like the spicule which occupied it. 
The earliest spicules arise on the second day of fixation, 
rather later than the mesenteries and before the tentacles, 
and may be examined with low powers of the microscope. 
They appear as small refringent nodules in the upper surface 
of the mound-shaped polyp, and show through by transmitted 
light. One row of small unbranched spicules is present in 
the body wall by the time the mouth opens (PI. 5, figs. 36 
and 37), and in older polyps they are found in the upper and 
lower regions of the body wall, the bases of the tentacles, 
and in the outer edge of the mesenteries very near the base 
(Text-fig. 14). According to Woodland, after the two 
daughter nuclei have formed in the scleroblast, the steadily 
growing round spicule lengthens and assumes a simple 
dumb-bell shape (PI. 4, fig. 31, No. 3). The ends of this 
dumb-bell become gradually elaborated into processes, so that 
the spicule then resembles a caudal vertebra in shape 
(Nos. 4 and 5). The colonies he utilised in following out the 
development of the spicule were small — about half an inch 
across — and the drawings he made have all been verified on 
the solitary polyp, with the exception of his figures illustrating 
the cavities which the spicules occupied. 
13. Mesenteric Filaments. 
Each mesentery bears a filament on its free inner edge 
running down for some distance from the lower opening of 
the stomodaeum (Text-fig. 48). While the two dorsal fila- 
ments create an upward current of water in the coelenteron 
by the active lashing of the cilia borne on the cells which 
cover them (Text-fig. 49), the six ventral filaments are 
secretory and absorptive. The ferment poured upon the food 
by these filaments continues and probably completes the 
disintegration begun in the stomodaeum. In the oldest 
solitary polyp examined (fixed from thirteen to fifteen days), 
the ventral filaments reached almost to the bottom of the free 
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