OP THE SPONGIAD^. 51 



skeleton. We may, therefore, reasonably conclude, tliat 

 their normal function is that of increasing the strength and 

 substance of the sarcodotis structure of the sponge. 



In the performance of this office of strengthening and 

 supporting the saroode, we find a singular class of spicula, 

 consisting of from three to six rays, emanating from a com- 

 mon centre, and always disposed at right angles to each 

 other. Between the extreme forms of development of these 

 and the simple stellate spicula, there is a very great amount 

 of structural difference ; but on a more intimate acquaint- 

 ance with the intermediate forms, we find them passing 

 into each other so gradually as finally to connect the whole 

 into one group. 



It is not in the Spongiadse only that these singular and 

 beautiful organs are found. In the soft parts of the 

 extensive famUy of the Gorgoniadae they are in vast 

 abundance, and in every variety of form, from an elongate 

 tubercular spiculum to the elongo-stellate forms of the 

 Spongiadse, and the prevalence of the bluntly terminated 

 radii is strongly indicative of their non-defensive character. 

 But this latter quality does not obtain in other cases, 

 either as regards the higher tribes of animals or the Spon- 

 giadse. Thus we find in numerous species of compound 

 tunicated animals their fleshy substance is crowded with 

 sphero-granulate spicula, very closely resembling in form 

 those of the sphero and subsphero-stellate shapes so abun- 

 dant in Tethea Ingalli and T. robusta (Figs. 164 and 165, 

 Plate VI). In both these cases the acute termination and 

 the peculiarities of their respective situations are indicative 

 of their subserving the office of defensive, as well as that of 

 consolidating spicula. 



Simple Stellate Spicula. 



Stellate spicula are composed of few or many radii 

 emanating from a centre in all directions. Their simplest 

 form is when the bases of the radii all proceed from a 

 common central point (Fig. 158, Plate VI), in which case 

 they should be designated simply, stellate spicula; but 



