14 ANATOMY AND PHYSIOLOGY 



nislied with spines. They are either irregularly matted 

 together, collected in fasciculi, or dispersed within or upon 

 the keratose fibres of which the skeleton is to a great extent 

 composed. Occasionally, but not frequently, they assume the 

 triradiate form. In the calcareous sponges, beside the simple 

 elongate form, the triradiate spicula are found in abundance. 



All the elongate forms of spicula of the skeleton are sub- 

 ject to extreme variety in length. In some species they 

 maintain a great degree of uniformity, while in others they 

 vary to a very considerable extent, according to the neces- 

 sities arising from the mode of the construction of the ske- 

 leton. When the areas of the reticulations are large, they 

 are generally long and rather stout, and are usually shorter 

 when the proportions of the network are small and close. 

 When enclosed in keratose fibre, they are most frequently 

 smaller and shorter in their proportions than those in the 

 Halichondroid sponges. And in those species in which 

 they are dispersed over the membranous tissues, as in 

 Hymeniacidon, Bowerbank, they are generally long, slender, 

 and frequently flexuous. In the sponges of this structure 

 having siliceous spicula the triradiate form of spiculum 

 occurs but rarely, while in the calcareous sponges, which 

 consist of membranes and dispersed spicula, the triradiate 

 forms of skeleton spicula are the normal ones. 



When the skeleton is constructed of large fasciculi of 

 spicula, as in Tetliea and Geodia, they attain their greatest 

 dimensions as essential spicula of the skeleton, frequently 

 exceeding the eighth of an inch in length. 



The greatest known length of spicula occurs in the pre- 

 hensile ones of Euplectella asperf/illum and cucumer, Owen, 

 where they are found to exceed three inches in length ; and 

 in HyaloRema mirabilis, Gray, where in the spiral column 

 of the great cloacal appendage they reach the extreme di- 

 mensions of six or seven inches in length ; but in both 

 these cases the spicula must be considered as auxiliary, and 

 not essential forms. 



The larger number of forms of skeleton spicula are per- 

 fectly smooth, but in some species they are partially or en- 

 tirely covered with spines. 



