BY E. F. HALLMANN. 401 



Skeleton. — The skeleton (Plate xxi., fig.l) is composed of well- 

 developed spiculo-spongin fibres, echinated, as well as cored, by 

 the characteristic inegascleres, the rhabdostyli : megascleres 

 scattered between the fibres are rare or absent. With respect 

 to its arrangement, it presents, in keeping with the lamellar 

 form of the sponge, a somewhat denser central, or axial region, 

 of usually fairly considerable width, within which the direction 

 of the main fibres is longitudinal or nearly so, and connecting 

 fibres between them are numerous; and exteriorly to this (on 

 either side of it, in the main body of the sponge), extending 

 therefrom to the surface, an e:rtra-axial region, in which the 

 direction of the main fibres is more or less obliquely transverse 

 (occasionally, in the lobes, nearly perpendicular to the longitu- 

 dinal direction), and connecting fibres between them (excepting 

 in the older portions of the sponge) are relatively few. The 

 demarcation between the two regions, however, is generally by 

 no means pronounced. In the extra-axial region, particularlv, 

 the skeleton is of very regular and simple pattern (Plate xxi., 

 tig 1), with the main fibres running in nearly straight courses, 

 approximately parallel to one another, at a distance apart of 

 about a spicule's length, and chiefiy connected together by their 

 echinating spicules in the manner presently to be described, 

 eventually, however, in the older portions of the sponge, these 

 connecting spicules become invested each with an ensheathing 

 layer of spongin, thus giving rise to connecting fibres, and the 

 pattern of the extra-axial skeleton is then rectangularly reticu- 

 late. In the axial region the main fibres are much less regularly 

 disposed, and the pattern of the skeleton is distinctly reticulate 

 throughout, with (except in the youngest portions of the sponge; 

 meshes of more or less rounded shape; in the oldest portions of 

 the sponge the meshes become reduced in size, often almost to 

 the point of obliteration. The spicules coring the main fibres 

 (both axial and extra-axial) are disposed, somewhat loosely, in a 

 slightly plumose fashion, frequently with the points of some of 

 them projecting a little beyond the spongin-sheath, and varv in 

 number, in a cross-section of the fibre at any point, from 3 to 

 about 8; they are generally somewhat the more numerous, and 



