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 ^ 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 - 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 fi 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 fj. 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 /^ in thickness of tyle and adjacent parts of the spicule 

 were only observed in rhabds less than 20 m 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 



' 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. 

 « H. V. Wilson. Mem. M. C. Z., 1904, 30, p. 52. 



