200 ANATOMY AND PHYSIOLOGY 



Suborder IV. Spiculo-fibrous skeletons. Regularly fibrous. 

 Fibres filled with spicula. 



Desmacidon. 



Raphynis. 



The spiculo-fibrous skeletons differ from the fibro-spicular 

 ones in this respect. In the first the form and proportions 

 of the fibre are dependent on the greater or the less de- 

 velopment of spicula, and the keratode serves only as a 

 cementing and coating material. In the latter the keratode 

 is the primary agent in the formation of the fibre, and the 

 spicula the secondary or auxiliary agent only. 



DESMACIDON, TSowerbank. 



HALICHONDBIA, Johnston. 



Skeleton fibrous, irregularly reticulated. Fibres composed 

 entirely of spicula arranged in accordance with the 

 axis of the fibre, cemented together and thinly coated 

 with keratode. 



Type, Desmacidon fruticosa, Bowerbank. 



The structure of the skeleton fibre in this genus readily 

 distinguishes it from all others. The form and size of the 

 tissue is entirely dependent on the greater or less quantity 

 of spicula present ; the keratode serving only as a cementing 

 and coating material. Halichondria wgagropila andfntti- 

 cosa, Johnston, are the only two British species of the genus 

 known. Fig. 264, Plate XIII, represents a fibre from the 

 skeleton of Halichondria (eyagropila, Johnston, illustrating 

 the structure of multispiculated keratose fibre, X 108. 



