184 ANATOMY AND PHYSIOLOGY 



the fasciculi of defensive spicula (a) and the mode in which 

 they are supported by buttresses of spicula beneath the 

 surface of the sponge at b; c> the recurvo-ternate spicula, 

 X 50 linear, 



HALICNEMIA, Bowerbank. 



Skeleton formed of a single superior stratum of spicula 

 radiating from the centre to the circumference of the 

 sponge at about its middle, and of an inferior stratum 

 of spicula distributed without order. 



Type, Halicnemia patera, Bowerbank. 



The nearest alliance to this genus appears to be Tethea, 

 in which the skeleton is formed of numerous fasciculi of 

 spicula radiating from the centre to all parts of a spherical 

 or elliptical mass ; while in Halicnemia the radiating fas- 

 ciculi are confined to a common plane, beneath which there 

 is a second stratum of spicula, which fills the space beneath 

 the radial stratum and the lower surface of the sponge, but 

 without being disposed in order; and the spicula of the 

 inferior stratum differ materially in form and proportions 

 from those of the superior one. 



In all the specimens of this genus that I have seen there 

 is a small pebble imbedded in the centre of each sponge, 

 from the surface of which the basal fasciculi of the radial 

 series emanate ; but although this appears to be the estab- 

 lished habit of this species, it is advisable not to consider 

 it as a generic character, although it may eventually prove 

 to be that the pebble is as much a portion of the skeleton 

 of the animal as the grains of extraneous matter which are 

 taken up by and become imbedded in the keratose fibres 

 of the genus Dysidea. Fig. 363, Plate XXXII, repre- 

 sents a portion of a section at right angles to the surface of 

 the sponge, exhibiting the mode of disposition of the 

 spicula of the skeleton X 25 linear. Eig. 364 is a view 



