68 ANATOMY AND PHYSIOLOGY 



the common honeycomb sponge of commerce, in the con- 

 dition in which it came from the sea. The primitive 

 fibrous tissue is seen arranged in a single layer in parallel 

 lines at right angles to the long axis of the canal, but 

 partially obscured by the stratum of sarcode on the 

 membrane. 



Fig. 256, Plate XII, represents a small portion of 

 the dermal membrane of a Stematumenift, in which the 

 primitive fibres are seen wandering in every direction over 

 the surface of the membrane. 



Figs. 257 and 258 in the same plate represent portions 

 of a stouter and a more compound membranous structure, 

 from the walls of one of the great cloacal projections from 

 the surface of Alcyoiiccllum rolmsta, Bowerbank. In this 

 case the membrane is strengthened by two or more layers 

 of primitive fibrous structure, the parallel fibres of each 

 crossing the others at various angles. 



FIBROUS STRUCTURES. 



There are two well-characterised classes of fibrous 

 structure : 



1st. Primitive fibrous tissue. 

 2nd. The fibres of the skeleton. 



1 . Primitive Fibrous Tissue. 



The first of these tissues is exceedingly minute. The 

 fibres are cylindrical in form, and are usually of considerable 

 length ; but where they are fully developed, they occur in 

 such numbers, and in such a matted condition, that I have 

 been unable to separate an unbroken one from the mass. 

 They continue through the whole of their length as nearly 

 as possible of the same diameter, and there rarely appears 

 to be any attenuation towards their terminations, which are 

 usually obtuse. They are evidently very elastic and 

 contractile. When partially separated from their attach- 

 ments to the membranes, the free ends seldom remain 



