373 



In the fourth part the fibrous tissues are described as consisting 

 of three principal divisions. 



1st. Primitive fibrous tissue. These structures are exceedingly 

 minute, and form an important element in the construction of the 

 compound membranous tissues of the animal. 



2nd division. The fibres of the skeleton are described under the 

 following heads : 



1st. Solid simple keratose fibre. 



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

 mary element. 



3rd. Multispiculate keratose fibre ; the spicula being the primary 

 element, and the keratode the secondary or cementing medium. 



4th. Iriequi- spiculated 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. 



7th. Regular arenated 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 

 cavities of sponges. 



PART II. Organization and Physiology. 



The author treats this portion of his subject under the following 

 heads : 



