100 THE SPONGES. 



5. Oxytylotes are occasionally met with. Their shape and size indicate 

 that they are derived from the long stout variety of oxea. 



Microsdercs. 



6. Plesiaster, Figs. 10 h, 10 d, Plate 13. The spicule is very abun- 

 dant in the parenchyma ; rays commonly 4 or 5 in number, 40-60 ju. by 

 4-5 /A, minutely roughened and tapering evenly to points ; centrum and 

 neighboring parts of rays smooth. Triactine, diactine, and monactine 

 forms occasionally occur. 



Smaller sizes with more numerous rays (Fig. 10 e, Plate 13) occur 

 intermingled with the larger characteristic forms. In these spicules, which 

 offer a transition to the metasters, the rays measure about 20 /a x 2 ju,. 



7. 3fetasters are abundant in the general dermal membrane and in the 

 pore membranes, and occur scantily in the parenchyma. The rays taper 

 to points and are most minutely roughened, while the axis is smooth. 

 The spicules differ among themselves a good deal, and fall into three 

 groups which intergrade. 



a. Typical metasters, Figs. 10 c, 10/, 11 a, 11 d, Plate 13, are fairly 

 common. The rays may be numerous, as in 11 «, or few, as in 11 d. Total 

 length of the spicule, 30-38 /a; rays, 12-14 /x x 2 ^u,. 



b. The predominant type of metaster is one in which the axis is very 

 thick and short (Figs. 10 a, 11 c, 11 e, Plate 13). The rays may be few 

 (Fig. lie) or many in number (Fig. lie). Spicule length, 28-30 fi; ray 

 length, 10-12 f^. 



c. The axis of the metaster may be greatly shortened, approaching the 

 condition of a centrum (Figs. 11 b, 11/, 11^). Such spicules may be desig- 

 nated metaster-oxyasters. The ray length is about 8 /x. 



Vosmaer (1902, p. 3) argues against the existence of transitional forms 

 between spirasters and true asters (euasters). It seems to me nevertheless 

 that the spicules here described may properly be considered as transitional 

 forms between metasters and oxyasters, even if (this appeared not to be 

 the case) such images as are represented in Figs. 11/ and 11 y are only end- 

 views of spicules like Ilk 



d. The metaster is, rarely, found varying toward the amphiaster, as 

 in Fig. 10 (/, Plate 13. The spicule may conveniently be designated 

 metaster-amphiaster. It has a few rays at each end, and one or two rays 

 on the axis. Spicule length, about 40 /x; ray length, about 18 /x. 



