Dr. J. S. Bowerbank on the Spongiadae. 421 



2ncl. Spiculated keratose fibre, in which the keratode is the pri- 

 mary element. 



3rd. Muhispiculate keratose fibre, — the spicula being the primary 

 element, and the keratode the secondary or cementing medium. 



4th. Incqui-spiculatcd keratose fibre. Consisting of skeleton 

 fibres constructed of numerous spicula irregularly dispersed, but con- 

 gregated into a round or oval massive fibre. 



5th. Simple fistulose fibre. A keratose fibre having a continuous 

 simple central canal. 



6th. Compound fistulose fibre. A keratose fibre with a continu- 

 ous central canal, from which secondary small canals branch at about 

 right angles to the primary one. 



rth. Regular areuated keratose fibre. Constructed of solid cylin- 

 drical fibres, in the centre of which there is a series of grains of 

 sand or other extraneous matters. 



8th. Irregular arenated keratose fibre. Consisting of grains of 

 sand or other extraneous matters cemented together into a continu- 

 ous cylindrical fibre. 



In the third division the siliceous fibrous tissues are described as 

 solid cylindrical structures, similar in form to the solid keratose fibres 

 of the second division, but consisting of pure silex in place of kera- 

 tode. 



The fifth part contains descriptions of the cellular structures. 



The sixth and last part of this division treats of the physical cha- 

 racters of the sarcode or semi-gelatinoid substance lining the interior 

 carities of sponges. 



Part II. — Organization and Physiology. 



The author treats this portion of his subject under the following 

 heads : — 



1st. The skeleton. Its general structure and component parts. 

 Under this head the physiological purposes of the various forms of 

 spicula, treated of in the first part of the paper, are described, and 

 their peculiar offices in the sponge pointed out. 



2ud. The sarcodous system is considered by the author as the 

 homologue of the mucous lining of the stomach and intestines of the 

 higher tribes of animals, and probably as the equivalent of the nervous 

 system also. 



3rd. The interstitial canals are considered as the equivalents of 

 the stomach and alimentary canals of the higher animals. 



4th. The intermarginal cavities, situated immediately beneath the 

 surface, and receiving the incurrcnt streams from the pores, are be- 

 lieved by the author to be the organs for the secretion of the vital 

 fluids of the animal. 



.5th. The dermal membrane, enveloping the whole of the sponge, 

 and in which the inhalant and exhalant orifices of the animal are 

 situated. 



6th. The pores or inhalant orifices. These organs are not per- 

 manent ; I. e. they are opened and closed at the will of the animal, 

 and when once closed seldom occur again in precisely the same spot. 



