120 THE SPONGES. 



combined with the body of the fibre, and there are always some free spicules 

 in the meshes. Individual fibres traceable only for short distances. Reticu- 

 lum undeveloped in spots, such places being occupied by a confused mass 

 of spicules. Meshes more or less rounded; diameter of the meshes about 

 equal to thickness of the fibres. 



In the cylindrical specimens longitudinal fibres are vaguely discernible 

 in the axial region. In all the specimens fibres directed more or less 

 radially to the surface, with tangential connectives, may be distinguished 

 in the superficial region of the body. 



The dermal membrane is supported by a reticulum (Fig. 2, Plate 21), 

 which is merely the outermost part of the main skeleton, and does not 

 differ essentially from the latter. It consists of rather poorly defined 

 fibres 400-600 /a thick, enclosing rounded meshes, the diameter of which 

 about equals the thickness of the fibres. Many of the meshes are nearly 

 free of spicules. Others are crossed by numerous scattered spicules, and 

 partly by spicule tracts. Although there are scattered over the surface 

 radially projecting spicules (only the points of which emerge), there are 

 no projecting tufts of spicules, such as in many Petrosia species are pro- 

 duced by the continuation of the radial fibres. 



On the smooth under surface of the two more or less plate-like forms, 

 the spicules of the dermal skeleton are thickly and irregularly scattered, 

 and not so arranged as to form a reticulum. In spots, however, this con- 

 tinuous layer of irregularly strewn spicules is interrupted by areas of 

 dermal membrane free of spicules, and containing, each, one or a few 

 pores. 



Comparaiive. In the type specimen of P. rariaUUs taken by the " Alert," 

 near Port Darwin, North Australia (Ridley, 1884, p. 415), the oxeas meas- 

 ured 400/x, X 19 /x. In the "Challenger" specimen of P. variabilis var. 

 from the Philippine Islands (Ridley and Dendy, 1887, p. 13), the oxeas 

 were 450 )u, x 22 /a. In the "L'Hirondelle" specimens of P. variabilis, from 

 the Azores (Topsent, 1892, p. 68), the oxeas were 530 /a x 33 /a. In the 

 " Belgica " specimens of P. variabilis, from the Antarctic Ocean (Topsent, 

 1901 a, p. 11), the oxeas were 535/a x 23 ;u,. 



As to the skeletal arrangement of P. variabilis Ridley (1884) says, "Main 

 skeleton — very loose primary lines of spicules, about three spicules broad, 

 runnino- irregularly towards surface, crossed by secondary tracts of similar 

 character, 2 or 3 spicules broad, at right angles to the primaries and about 



