and Physiology of the Spongioid. 9 



lines of large fibre can be distinguished ; and thus a simple 

 reticulation goes on from the base to the circumference, pre- 

 senting a simple gradation in size from the oldest to the latest- 

 formed portions. 



The fibre may be glass-like, horny, or spiculous — that is 

 (in the latter case), composed almost entirely of spicules bound 

 together by a minimum of sarcode. 



(Spicules are siliceous or calcareous bodies, according to 

 the nature of the sponge, which are developed by the sponge 

 itself, and vary greatly in form, being for the most part linear 

 and pointed at each end, as will be more particularly described 

 hereafter.) 



Again, the glass-like fibre contains a core of spicules ; and 

 the horny may be cored with a fine granular substance or with 

 foreign bodies or spicules respectively. 



Thus the fibre consists of two distinct parts, viz. the wall 

 and the axis or core. 



There is no difficulty in distinguishing between the glass- 

 like and the horny fibre ; but there is frequently a difficulty 

 in determining between the horny cored with proper spicules 

 and the spiculous fibre, since the horny substance is com- 

 posed of the same material as the film which binds together 

 the spicules of the spiculous fibre ; and therefore the distinc- 

 tion is only one of degree, viz. that of whether the spicules 

 or the horny substance forms the chief part of the fibre. 

 Still, for the sake of classification, it will be found by-and- 

 by necessary to make the distinction. When horny, the 

 horny matter preponderates ; when spiculous, the spicules. 



The axis or core, however, is evident in all. Thus in the 

 glass-like fibre it consists of proper spicules (" proper spicules " 

 are spicules that are formed by, and peculiar to, the species) ; 

 while in the horny fibre the core may consist of a fine, uni- 

 formly granular, tubular membrane or sheath everywhere 

 anastomosing and the same, or of foreign objects which, in 

 some parts, may be so scanty that the fibre for the most part 

 is horny throughout ; or the core of foreign objects may be 

 so general as to form the axis in every part of the fibre, so 

 that there are many degrees between these two extremes ; or 

 the core may consist of foreign objects and " proper spicules " 

 mixed together, or of proper spicules alone. Lastly, as before 

 stated, in the spiculous fibre not only the axis, but the whole 

 fibre is composed of " proper spicules " held together by a 

 minimum of hardened sarcode, which from its thinness is 

 almost imperceptible, while the fibre thus composed is, when 

 dry, opaque and white. 



In addition to the core, the fibre is sometimes echinated 



