SPONGES 



155 



of lloeiucr, the typical spicule (niegasclere) has eiglit rays (Fig. 95, A). Six 

 of the rays are i)laced in one jilane, wliioh may be tenned horizontal, 

 and in Avhich they radiate out 

 at equal angles of 60° from a 

 eoniniou 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 ab-sent alto- 

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



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

 six to thii-ty rays, coming off from a common centre at different angles 

 (Fig. 96, ^1). This type form is again less common than some of its 

 modification.?. 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), 



Spicules of Ast rn.eospongin. A, octactine ; 

 JB, hexactine. (After Hinde.) 



-Z>. 



Fig. 96. 



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

 like fonns— C'and H in protile, D from below. (After Hinde.) 



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

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

 this type produces nail-like spicules (Fig. 96, C, I>, E), in which there is a 

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

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



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