OF THE SPONGIADiE. 77 



line from the parent canal ; a few assume a tortuous 

 direction, and a still fewer number bifurcate or branch. 

 Within the central tubes of the fibres there are frequently 

 one or two minute simple tubular fibres j when more than 

 one they do not unite, but they divide and traverse each 

 a separate cavity, when they happen to reach one of the 

 anastomosing points of the great skeleton-fibre. The 

 structures are described more at length in the ' Annals and 

 Magazine of Natural History,' vol. xvi, p. 405, under the 

 head of " AulisMa," a new genus of sponges, founded prin- 

 cipally on the compound fistulose structure of its skeleton- 

 fibres. Pig. 268, Plate XIV, represents a portion of com- 

 pound fistulose keratose fibre as seen with a linear power 

 of 100. Pig. 267, Plate XIII, a portion of a similar fibre 

 under a power of 300 linear. 



8. Regular Arenated Keratose Fibre. 



This description of fibre under ordinary circumstances 

 has very much the appearance of simple fistulose fibre, but 

 when examined by transmitted light with a linear power of 

 about 100 we find in the centre of the fibre a series of 

 grains of extraneous matter, occupying the place of the 

 large continuous canals of the fistulous forms of fibre. 

 The series of extraneous matters is not always con- 

 tinuous, and when an interruption takes place the fibre 

 becomes solid, or faint traces only of a central cavity remains. 

 The mode of the inclusion appears to be due to the extreme 

 terminations of the young fibres being viscid, and thus 

 seizing on any extraneous particles that happen to come 

 in contact with them. The growing keratode quickly 

 envelopes them, and, proceeding on its course of extension, 

 seizes in like manner on other particles of sand or soHd 

 matter, and thus a continuous and regular chain of extrane- 

 ous material is imbedded in the axis of the fibre, as repre- 

 sented by Pig. 269, Plate XIV. This description of fibre 

 is found in a great variety of keratose sponges, and especially 

 so among the coarse rigid skeletons of the Australian species. 

 And among the flexible sponges, as represented by Pig. 269. 



