410 STRUCTURE AND ECONOMY OF TETHEA. 



spieula are arranged symmetrically so as to circumscribe the pe- 

 culiar polygonal figures which must result therefrom. I have, 

 however, hitherto failed, either because the eye is unable in 

 the maze of crossing spicular branches to isolate the pattern ; 

 or because they have been disarranged, as I am inclined to be- 

 lieve they are, first, in consequence of the varied development 

 of the intervening soft textures, the result of varied conditions ; 

 and secondly, in consequence of the oblique course of the 

 water-passages which they support. 



In addition to this arrangement of radiated spieula, the 

 peripheral portion of the skeleton consists of the divergent 

 naked, or imbedded spieula of the terminal branches of the 

 central portion. The greater number of these appear to me 

 to pass on or through the spaces inclosed by the rectangular 

 secondary branches of the six-radiate spieula ; and from this 

 circumstance, as well as from the fact of the projecting but 

 still imbedded spieula forming conical tubercles or thin spaces, 

 I am inclined to consider these spaces (which are also, from 

 the form of the six-radiate spieula, somewhat elevated) as the 

 parts of the surface set aside for the cutaneous attachments 

 of the central portion of the skeleton, and as homologous in 

 this new form of Tethea to the conical elevations of T. cranium, 

 or hemispherical bulgings of T. lyncurium. 



I have already stated the general characters of the different 

 kinds of spieula which I have observed in this sponge, but 

 the very interesting nature of these silicious structures de- 

 mands a few additional remarks. 



Into the consideration of their chemical composition I 

 shall not at present attempt to enter. 



I am rather anxious to direct attention to their exact con- 

 figuration and to their mode of development. 



From the comparatively large size of the elongated spieula 

 of this species of Tethea, I am strongly confirmed in an 

 opinion which I have long entertained from the examination 



