94 THE SPONGES. 



5. A number of very slender, nearly cylindrical spicules, clavate at one 

 end, as shown in Fig. 6, Plate 12, occur in the roots. The spicule just 

 below the clavate end is 16 ju. thick, narrowing then to 12 /i, then very 

 gradually increasing in diameter for a length of 6 or 7 mm. to a thickness 

 of 24 /i,, at about which point all the spicules observed were broken across. 



Microsderes. 



6. Spirasters, Fig. 3, a, h, c, Plate 12. Spirasters are abundant, but 

 not crowded, throughout the parenchyma and ectosome, including the 

 general dermal membrane, and thickly scattered in the pore membranes. 

 There are no constant differences between the spicules of the several 

 regions. Total length of the spicule, 28-40 /a ; ray length, 10-14 jx. The 

 larger sizes are more common in the parenchyma and ectosome than in 

 the pore membranes. 



The spiral axis is smooth, and varies in length, so as to exhibit in 

 projection one or two concavities on the same side. The axis in different 

 spicules varies in thickness. In some it is slender, 2 /x thick, and seems to 

 be cylindrical in shape. Very commonly the axis is flattened and band- 

 like, hence appearing wider in one part of the spicule than elsewhere 



(Fig. 3 c, Plate 12), reaching in such parts a width of 4-5 /a. The 



rays appear rather long and slender, are minutely roughened and minutely 

 capitate, tapering, numerous, and closely set. 



A number of deviations from the type are met with, represented by a 

 few spicules found here and there. Among these a form approaching the 

 amphiaster occurs, consisting of a straight bar, at each end of which rays 



are clustered in a spiral curve. Spirasters also occur, in which the 



rays are longer, fewer in number, and less closely set along the axis than 

 in the type. Such a spicule had a total length of 40 /a ; rays 12 in number 

 and \^ jjL long. 



