30 ANATOMY AND PHYSIOLOGY 



ternate apices are capable of such an amount of oscillating 

 motion, as would be required for the organic expansion and 

 contraction of the membranous structure they support. 

 By the action thus generated each pair of the united radii 

 would glide in a longitudinal direction upon each other, and 

 thus, although in each separate instance the amount of 

 motion would appear to be exceedingly small, the aggregate 

 of the whole would afford a very considerable range of 

 expansion, as exhibited in Pig. 306, Plate XX. 



In their second mode of application, that is to the bases 

 of the intermarginal cavities, it appears that as their office 

 is different, so their form, and the mode in which the radii 

 of their apices is connected is also different. Thus at the 

 inner surfaces of the thick dermal crust of Geodia 

 McAndrewii and Barretti, we find them forming a network 

 equally regular and continuous as that in Dactyloccdyx 

 Prattii, but the mode of its construction is varied. The 

 radii do not in these cases glide upon each other longitu- 

 dinally, but they cross each other at various angles ; and as 

 the whole mass of these sponges are fleshy and very elastic, 

 so by this mode of interlacement of the radii a very consi- 

 derably greater amount of expansion and contraction of the 

 reticulated structure is provided for, while at the same time 

 the power of maintaining the common plane of the reticulated 

 tissue is equally as great as in the similar structure in 

 Dactylocalyx Prattii. Thus far we can trace the physiolo- 

 gical purpose of their structure; but why in one species 

 we find their terminations simple as in Geodia McAn- 

 drewii, and furcated as in Geodia Barretti, or still further 

 complicated as in the dichotomo-patento-ternate form, is 

 a question which cannot be so readily solved without a 

 further acquaintance with the species of Geodia bearing 

 these forms in a Hving state. 



Prehensile Spicuta. 



Spicula projected from a sponge as a means of attachment 

 to other bodies.— I know of but one form of this description 

 of spiculum, an exceedingly elongated, . fusiformi-acerate 

 one, with a stout recurvo-quartemate apex. It occurs at 



