76 ANATOMY AND PHYSIOLOGY 



diameter. In the young state the cavity is as large, or 

 nearly so, as in the adult fibres, while the enveloping 

 keratode assumes the form of a thin, transparent, amber- 

 coloured coat, which in the mature state becomes frequently 

 twice or three times the thickness of the diameter of the 

 central cavity. 



This great fistular space is lined with a thin pellucid 

 membrane, which, in specimens that have been dried, 

 appears to have been thickly covered with minute semi- 

 opaque granules. At the time of my first description of this 

 form of fibre, published in the ' Annals and Magazine 

 of Natural History/ vol. xvi, p. 403, I believed that in 

 the natural condition of the fibres the central cavity was an 

 open tube, but subsequent observations on specimens which 

 have never been dried have led me to the conclusion that 

 the whole of the central space is filled with a minutely 

 granulated substance, which presents all the characteristics 

 of sarcode. 



There is no communication between the great central 

 fistular canal and the interstitial cavities of the sponge, the 

 projecting ends of the fibres of the skeleton being always 

 hermetically sealed. Eig. 2G6, Plate XIII, represents a 

 fibre from the specimen of Spongia Jistidaris, Lamarck, in 

 the Museum at Edinburgh, given to me by Prof. Grant. 



7. Compound Fist dose Keratose Fibre. 



In its external characters this description of fibre is not, 

 under ordinary circumstances, to be distinguished from 

 the simple fistulose fibre, and it is only when submitted to 

 a microscopical power of about 100 linear that its peculiar 

 character can be detected. We then find that the fibre is 

 not only furnished with a large continuous central cavity, 

 but that it also has numerous minute caecoid canals 

 radiating from the central one at irregular distances, at 

 nearly right angles to its axis. These secondary canals are 

 very unequal in length, and very few of them reach to near 

 the external surface of the fibre, and none of them appear 

 to perforate it. Their direction is usually in a straight 



