53 ANATOMY AND PHYSIOLOGY 



when the radii spring separately and distinctly from a 

 common central spherical or oval base, they should be 

 designated sphero-stellate spicula (Figs. 163, 164, 165, 

 166, 167, Plate VI). In both these classes of spicula there 

 is a very considerable difference in their size and form, in 

 the various species of sponges in which they occur. 



Compound Stellate Spicula. 



The curious and beautiful forms of this series of spicula 

 all belong to the class of sponges that have a skeleton 

 composed of siliceous fibre, and they are principally from 

 tropical climates. The central basal structure from which 

 the radii are projected, in every case with which I am 

 acquainted, is a rectangulated hexradiate spiculum, from 

 the apices of which a variety of beautiful terminations are 

 projected, which vary in form exceedingly in different 

 species of sponges. In the class of sponges to which I 

 have alluded there are also numerous rectangulated spicula, 

 var3ring in the number of radii from three to six, the 

 apices of the radii being either acutely terminated or more 

 or less elevated, and these forms vary very much in size. 

 They are unconnected with the skeleton, and evidently 

 belong to the Sarcodous system of the sponge. They are 

 very much larger than the hexradiate centres of the com- 

 pound stellate spicula, but as they are evidently the normal 

 forms of that tribe, I shall describe the general characters 

 of these large, simple, hexradiate forms before those of the 

 more complicated stellate ones. 



Attenuated rectangulated hexradiate (Pig. 174, Plate. 

 VII.) — The first state in which we find them is in 

 that of an inequi-acerate spiculum (Fig. 175), in which 

 condition they are in fact the two axial radii of the 

 hexradiate form which they ultimately attain when in 

 their fullest state of development. In the next stage we 

 find a bud-like projection issuing from the side of the 

 thickest portion of the inequi-acerate spiculum (Fig. 176), 

 which is ultimately developed in the form of a rectan- 

 gulated triradiate spiculum, as in Fig. 179. Or two buds 



