96 
BATHYDORUS LAEVIS SPINOSISSIMUS. 
ray. These gastral spicules do not lie quite so close together as the dermals. 
The prostals which protrude from both surfaces are large and small rhabds. 
Besides the spicules described above, which I have observed in situ in the 
sections, some other forms, which I am inclined to consider as proper spicules of 
the sponge, also occur in the spicule-preparations. These are : — hemioxy- 
hexasters; angularly bent diactine megascleres; hexactine megascleres with 
fairly equal rays; hexactine megascleres, with one ray much longer than the 
other four; and pentactine megascleres, with relatively short lateral rays. The 
hemioxyhexasters, which are similar to the oxyhexasters, doubtlessly form part 
of the skeleton of the interior. The angularly bent diactines, and the hexactines 
with rays fairly equally long, may also take part in the formation of the interior 
skeleton. According to Schulze 1 such hexactines occur in the choanosome of 
the type of Bathydorus laevis. About the hexactines with one long ray and the 
pentactines with long proximal ray I have my doubts. Wilson 2 says that 
hexactines with one elongated ray, 10 mm. long, occur in Bathydorus laevis 
spinosus and that these spicules are here so situated that their elongated ray 
protrudes freely beyond the surface, prostal-fashion. The hexactines with one 
elongated ray observed by me were much smaller and made rather the impres- 
sion of being derivates of hypodermal pentactines, with a short apical distal 
ray. The pentactines with short lateral rays are probably also hypodermal. 
The rhabds (Plate 14, figs. 1-10) vary exceedingly in size, and a continuous 
series of intermediate forms connects the smallest with the largest. They are 
1-21 mm. long, and 5-105 p thick at the thickest point. The small rhabds are 
distinctly centrotyle (Plate 14, figs. 5, 6), many of the large ones without a 
central tyle. The four rudimentary rays which compose the tyle are often very 
clearly distinguished, particularly in the small rhabds. Not infrequently they 
are unequal in length, in which case the tyle formed by them appears eccentric 
(Plate 14, fig. 5). The tyle may measure 22 p more in transverse diameter than 
the adjacent parts of the spicule. This difference is not only relatively but also 
absolutely greater in the small and slender than in the large and stout rhabds. 
Differences of over 11 n in thickness of tyle and adjacent parts of the spicule 
were only observed in rhabds less than 20 p thick. 
The end-parts of the rhabds are conic (Plate 14, fig. 7), cylindroconic 
(Plate 14, figs. 7, 9), or cylindrical (Plate 14, figs. 1, 3, 10), and terminally 
1 F. E. Schulze. Hexactinelliden des Indischen Oceans, II. Abh. Akad. Berlin, 1895, 1896, p. 58, 
taf. 6, fig. 2; Indian Triaxonia, 1902, p. 79, pi. 14, fig. 2. 
2 H. V. Wilson. Mem. M. C. Z., 1904, 30, p. 52. 
