BY E. F. IIALLMANN. 529 



the extreme apices of the branches, the demarcation between the 

 axial region of the skeleton and the extra-axial, as seen in longi- 

 tudinal section, is very pronounced (more especially if the spongin 

 has been stained) notwithstanding there is no discontinuity 

 between the main fibies of the two regions (i.e., between the 

 longitudinal fibres of the former and the " radial " fibres of the 

 latter), such as might be inferred from the terms "primary" and 

 "secondary" used in the original description to distinguish them. 

 The contrast is partly due to the much greater density of the 

 axial skeleton, and also partly (i.)to the rapidity with which the 

 fibres change in direction fi'om longitudinal to almost perpen- 

 dicularly transverse, and (ii.) to the sudden and very considerable 

 diminution in the amount of their C(mstituent spongin,— as they 

 pass from the one region to the other; but there are other differ- 

 ences also. 



In the axial i-egion, the main or longitudinal fibres, which 

 have a maximal stoutness of 80 or 90/x, usuall}' contain multi- 

 serial spicules, for the most part not very conipacth* or regularly 

 arranged; are rather closely juxtaposed, and frequently coalesce 

 with each other for short distances; and are connected at close 

 intervals by short, aspiculous, transverse fibres. Participating 

 in the formation of the axial skeleton also are many spicules 

 whose relation to the fibres is more or less indefinite, as well as 

 many transversely and obliquely directed ones occurring singly. 

 In the older portions of the skeleton, the meshes of the reticula- 

 tion become much reduced in size, often to the point of oblitera- 

 tion, through the continued growth in stoutness of the fibres. 

 The more peripherally situated of the main fibres run, not longi- 

 tudinally, but with a slight, and gradually increasing, trend 

 outwards; ultimately they pass into the extra-axial region, and, 

 curving surfacewards, immediately subdivide each several times 

 in rapid succession to form the radial fibres. 



The radial fibres, throughout the greater part of their length, 

 are only two or three spicules broad; the spongin cementing 

 their spicules is usually so small in quantity as scarcely to be 

 discernible unless stained; and the connecting fibres between 

 them occur only at comparatively wide and irregular intervals. 



42 



