OF THE SPONGIAD.E. 57 



slender and complicated but elegant forms of the floricoino- 

 stellate spicula, and it is probable that their office is purely 

 that of assisting in the consolidation of the sarcodous 

 substance. 



The whole of these beautiful stellate forms of spicula are 

 siliceous, while their homologues in the Gorgoniadae and 

 the compound tunicata are calcareous ; and it is somewhat 

 remarkable that hitherto none of these forms have been 

 found in the calcareous species of sponges. 



Spicula of the Ovaria and Gemmules. 



We find the same laws in force regarding the spicula in 

 the structure of the minute bodies which have been 

 designated gemmules by previous writers on the Spongiadse, 

 that obtain in the sponges themselves. In some they 

 serve the purposes of internal skeleton and defensive 

 spicula as well. In others they combine the offices of 

 tension aud defensive organs, and frequently they are very 

 different in form from those of the parent sponge. They 

 occur in various modes of disposition. 



1. Those which have the spicula disposed at right angles 

 to lines radiating from the centre of the ovarium to its 

 surface. 



2. Spicula disposed in lines radiating from the centre to 

 the circumference of the ovarium. 



3. Those having the spicula disposed in fasciculi in 

 the substance of the gemmule from the centre to the 

 circumference. 



In the first mode of disposition they are sometimes 

 of the same form as those of the skeleton, but considerably 

 less both in length and diameter, to adapt them to the 

 office they have to perform. In other cases they differ 

 materially in both size and form from those of the 

 surrounding skeleton ; but in every case with which I 

 am acquainted, their long axes are parallel to the outer 

 surface of the case of the ovarium, or to the surface of 

 the ovarium itself. 



In the second mode of disposition they are immersed in 



