SPONGES 



155 



of Roemer, the typical spicule (megasclere) has eight rays (Fig. 95, A). Six 

 of the rays are placed in one plane, which may be termed horizontal, 

 and in which they radiate out 

 at equal angles of 60 from a 

 common centre. The two re- 

 maining rays radiate from the 

 centre in opposite directions, 

 forming a vertical axis which 

 cuts the horizontal plane at right 

 angles. Spicules of this normal 

 type are, however, less frequent 

 than a modification in which 

 the two vertical rays are reduced 

 to nodules or are absent alto- 

 gether, thus producing a flat, six-rayed star (Fig. 95, B). 



In the Heteractinellida the typical spicule is a huge euaster with from 

 six to thirty rays, coming off from a common centre at different angles 

 (Fig. 96, A). This type form is again less common than some of its 

 modifications. By the rays being placed nearly in one plane, in which 

 they are confluent at their bases, a disc-like star is produced (Fig. 96, B), 



Fio. 95. 



Spicules of Astraeosponyia. A, octactine ; 

 B, hexactine. (After Hinde.) 



r 



FIG. 96. 



Spicules of Heteractinellida. A, typical polyactine ; B, rosette-like form ; C, D, E, nail- 

 like forms C and E in profile, D from below. (After Hinde.) 



which may further have three or four rays coming off at right angles, or 

 nearly so, from one surface of the disc. A characteristic modification of 

 this type produces nail-like spicules (Fig. 96, (7, D, E), in which there is a 

 disc with six to nine rays projecting horizontally, from the centre of which 

 a stout ray is given off in a vertical direction. The rays may be equal or 



