206 Dr. J. E. Gray on the 



7. Birotulate spicules, consisting of a cylindrical axis, which 

 is furnished with a more or less circular disk at each 

 end. 



1. The needle-like, simple, subcylindrical spicule is the 

 most usual, and is present in its various modifications in almost 

 every spiculiferous sponge, very often by itself, but at other 

 times in combination with one or more of the other kinds of 

 spicules. 



The spicules are generally cylindrical, slender, and elon- 

 gate ; but they are sometimes short and thick. 



They generally become thinner at one end, like a needle 

 (British Sponges, t. i. f. 8) ; but they often taper at both ends, 

 as in ' British Sponges,' t. i. f. 1, 2, 3. 



Some spicules are like a pin and have a more or less 

 distinct rounded head at one end (B. S. t. i. f. 23, 24, 25, 26). 

 Sometimes instead of one head they have several globular 

 projections (B. S. t. x. f. 228 to 233). 



Some are swollen at each end or biclavate (B. S. t. i. 

 f. 18, 19, 20, & 27). 



In general the spicules are smooth ; but they are sometimes 

 partially or entirely covered with small asperities (B. S. t. i. 

 f. 28-30, 32, 33, t. iv. f. 90, 91, 93, t. ix. f. 204-207). 



The surface of the simple spicules is often more or less 

 tubercular or spinulose in part or over the whole surface (B. S. 

 t.i.f. 28-32). 



Sometimes. the spines on the spicules are placed in whorls 

 (B. S. t. i. f. 33, t. iii. f. 67, 68, t. x. f. 238). 



Sometimes the tubercles form conical elongate spines (t. i. 

 f. 35, t. iii. f. 72). 



Sometimes these spines, instead of being scattered, are placed 

 in whorls or transverse circular lines (B. S. t. i. f. 33, t. iii. 

 f. 66-68, 70, t. x. f. 238, 239, t. xi. f. 244-246, t. xvii. 

 f. 289-291,1 xviii. f. 296). 



Sometimes the tubercles on the surface are very long and 

 spinose ; but these may belong to another form (B. S. t. ii. 

 f. 41, t. iii. f. 69). 



The simple, fusiform or cylindrical spicule sometimes has 

 a subcentral belt (B- S. t. i. f. 4, t. iv. f. 95) ; but this must 

 not be confounded with the very much dwindled simple form 

 of the sexradiate spine (B. S. t. iii. f. 65), which has the cen- 

 tral thickened belt marked with four internal radiating canals. 



2. The hamate spicules are characterized by being unilateral, 

 and having most generally three, but sometimes only one well- 

 developed recurved hook at each end of the spicule ; but those 

 that are attached (and they sometimes form groups and are 

 fixed to the harder parts of the sponge) have the end 



