OF THE SPONGIAD/E. 17 



connection with the mass of the animal beneath. The trira- 

 diate apices also serve to construct areas in which are 

 situated the proximal orifices of the intermarginal cavities, 

 which are imbedded in the crustated surface of the sponge. 

 The normal form of these spicula is very different from that 

 of the spicula which constitute the general mass of the 

 skeleton, and they are far more complex and varied in their 

 structure. They usually have a long, stout, cylindrical, or 

 attenuated shaft, terminating either acutely or hemispheri- 

 cally at the base, while the apex is divided into three stout 

 equiangular radii, which assume in different species a con- 

 siderable amount of variety as regards form and direction. 

 The triradiate apices are usually cemented firmly to the 

 inner surface of the crustated coat of the sponge, while the 

 stout and elongated shaft is intermingled with and firmly 

 cemented by keratode to the general mass of the skeleton. 

 From the trind nature of the apex, I have designated these 

 forms as ternate spicula, prefixing such terms as may best 

 serve to distinguish them individually in accordance with 

 their permanent variations from each other. The prefixed 

 designations of the spicula must necessarily in some mea- 

 sure be arbitrary, as the differences in the degree of the ex- 

 pansion of the radii cannot be strictly defined; and although 

 the forms are well characterised in each species, yet even 

 within these bounds a slight degree of variation, arising 

 from the local necessities of the case, \vill occur. The ter- 

 nate spiculum, therefore, as a general designation, may be 

 said to be an elongate spiculum, with a triple apical ter- 

 mination. These spicula are not confined to the office of 

 connecting only, but are also found among the defensive 

 ones, as will be hereafter shown they are best developed in 

 Geodia McAndrewii and Barretti, Pacliymatisma Johnstonia, 

 and others of similar structure. 



I have never seen the progressive development from a 

 simple elongate shaft of an expando or patento-ternate con- 

 necting spiculum, as I have those of the porrecto-ternate 

 external defensive form, and the spinulo-recurvo-quaternate 

 internal defensive ones, but from the great similarity that 



exists in their structure there can be little doubt that their 



2 



