178 



NOTES AND MEMORANDA. 



if accompanied by increased concentric growth of the initial coll, then 

 the pin-headed acuate (Fig. 4) is produced. 



4. An elongation of the cell in two directions, inclined to each 

 other at a less angle than 180^, gives us the curved acerate (Fig. 5), 

 which is both biradiate and biaxial. 



fl 2 3 4 



As 11 6 7 8 



5. The inclination of the two rays in Fig. 5 is followed by the 

 appearance of a third in Fig. 6, where we have the triradiate spicule 

 of Plectronella. In this spicule two of the radii arise from the proximal 

 face of the cell and grow inwards towards the axis of the fibre on which 

 it is situated, and the third ray arises distally and grows outwards 

 away from the axis. 



6. A growth of the cell in three directions making equal angles 

 with each other, and having no determinate relations to any sym- 

 metrical line within the sponge, gives us the equiangular triradiate 

 spicule (Fig. 7), which occurs abnormally in Dercitus Bucldandi. 



7. A quadriradiate growth of the cell in directions having no 

 determinate relations to the form of the sponge gives us the normal 

 spicule of Dercitus Bucldandi (Fig. 8). 



8. The cell gives oif three buds from its distal face, which grow 

 outwards away from the sponge, and a fourth from its proximal face, 

 which grows inwards, and we have the forked forms of Geodia and the 

 like (Fig. 9). 



9. The cell grows in five directions along three axes at right 

 angles to each other, which are not determinately related to any lines 

 of reference within the sponge [Dercitus BucMandi), or which are so 

 related {Euplectella and other Hexactinellids), and we have the quinque- 

 radiate form (Fig. 12). 



10. In Fig. 6 the growth of the three rays is along directions 

 inclined somewhere about 120° with each other ; if two of the rays 

 grow in opposite directions, and the third at right angles to them, 



