62 ANATOMY AND PHYSIOLOGY 



those of either of the preceding groups. They are in 

 reality but modifications of the external defensive spicula of 

 the parent sponges. 



The inequi-fusiformi-acerate one (in the Gemmule of 

 Tethea) differs from the fusiformi-acerate one of the 

 skeleton in no other respect than in the greater propor- 

 tionate attenuation towards its distal termination, which 

 gives it a degree of flexibility that allows of its bending 

 freely under the pressure of any comparatively large body j 

 and I have seen them, when two gemmules have been 

 pressed closely together, bent to the extent of semicircles 

 without breaking. In the young gemmules these spicula 

 are usually projected much beyond the other forms of 

 defensive spicula that accompany them. 



In like manner the small attenuato-porrecto-ternate 

 form in the same gemmule is a modification of the similarly 

 formed external defensive spicula of the parent sponge. 

 In the adult gemmule the apices of these spicula rarely 

 project beyond the dermal membrane, and it is only on 

 pressure from without that they would be brought into 

 effective use. The amount of the angle of their radiation 

 at the apex of the spiculum. is therefore greatly increased 

 beyond those of the external defensive ones of like form in 

 the parent sponge, so as to accommodate their apices to 

 the curve of the surface of the gemmule, and to render 

 each point equally effective ; and as they are not projected 

 beyond the dermal surface, as in the sponge, their shafts 

 are shortened proportionally. 



The unihamate, bihamate, and recurvo-ternate forms of 

 the same gemmules are also modified forms of the recurvo- 

 ternate external defensive spicula of the parent sponges, 

 Tethea cranium and similima. 



KERATODE 



Is the substance of which the horny elastic fibres of the 

 skeleton of the ofiicinal sponges of commerce are composed 

 It has, correctly speaking, no relationship either chemically 



