126 A MONOGRAPH OF THE 



dispersed over the sponge without any connection with the 

 pustular organs. The dermal membrane is furnished 

 abundantly with tension spicula of the same form and size 

 as those of the columns of the skeleton, intermixed with 

 others of the same form, but very slender ; and it has also 

 a considerable number of retentive spicula intermixed with 

 the tension ones. The general aspect of this organ is very 

 much confused, in consequence of the terminations of the 

 skeleton columns frequently curving in the direction of 

 right angles to their long axes, and then spreading irregu- 

 larly over the inner surface of the membrane, so as to pro- 

 duce the appearance of its being furnished with irregular 

 flat fasciculi of tension spicula. This irregular habit of the 

 skeleton columns is, to the best of my belief, peculiar to this 

 species of Microciona. The skeleton columns are very 

 irregular in their forms and directions ; they are constructed 

 of numerous spicula. disposed longitudinally, and are 

 closely cemented together by keratode. The large external 

 defensive spicula are based on their surfaces, whence they 

 radiate at a great variety of angles, and not infrequently 

 at right angles to their axes. The small internal defensive 

 spicula are not very numerous, they are irregularly dis- 

 persed in all parts of the tissues, and are more frequently 

 recumbent on the interstitial membranes than erect. The 

 retentive spicula are small, but stoutly constructed ; the 

 lateral teeth are widely spread and slightly palmated. The 

 interstitial membranes are numerous and strong, and abun- 

 dantly spiculous, and exhibit on their surface, in a greater 

 or less state of development, every form of spiculum that 

 is found in the sponge, and the retentive spicula are espe- 

 cially numerous in some parts of them. Nearly the whole 

 of the characters exhibited by the interstitial tissues are 

 invisible until the section has been mounted in Canada 

 balsam. 



