138 THE SPONGES. 



through parenchyma, becoming fairly abundant in places. They vary 

 greatly in size, ranging from 64 to 22 /a in length. The microscopic 

 pictures afforded by the spicules differ a good deal in appearance, but the 

 spicules all have essentially the same shape. 



Skeletal Arrangement. The main skeleton (Figs. 2 and 3, Plate 22, 

 sections radial to the surface) is loose, consisting of irregularly disposed 

 tracts of spicules and scattered spicules. The larger-sized spicules, 800 /^ 

 and upward in length, predominate. In the superficial region numerous 

 tracts, extending more or less radially to the surface, are distinguishable. 

 The spicules are cemented together here and there by very small amounts 

 of yellowish and distinctly stratified spongin. 



The dermal membrane of the smooth, non-porous regions is densely filled 

 with tylostyles, disposed tangentially or slightly obliquely to the surfiice, 

 and forming several layers (Fig. 3, Plate 22 ; lower part of Fig. 2, Plate 

 22). As in the main skeleton the larger-sized spicules predominate. 



In the rough, porous regions, the radial or obliquely radial tracts expand 

 to form superficial brushes of small tylostyles, which project beyond the 

 surface (upper part of Fig. 2, Plate 22). The spicules of the brushes 

 measure for the most part 500 to 290 /x in length. Some of the brushes 

 project radially from the surface, but many project so obliquely as to lie 

 almost flat. The flat brushes, in which the spicules diverge widely, and 

 which consequently present a fan-like appearance, point in all (tangential) 

 directions, and in places cross one another to some extent. 



Comjmrative. Tylodesma alba resembles in different points several of the 

 species from the Florida coast briefly described by Schmidt (1870, p. 53) 

 under the name of Desmacella. Thus, as in DesmaceUa inimilio 0. Schm., 

 the tylostyles are " theils geschichtet, theils in Fasern und ragen mit den 

 Spitzen hervor." The smooth dermal membrane found over a large part of 

 the surface corresponds to that described for D. vagdbunda 0. Schm. Flat- 

 tened brushes of spicules similar to those present over the rough parts of 

 the surface in Tylodesma alba are mentioned by Schmidt as characteristic of 

 Desmacella vicina : " mit flachen, oft fjicherigen Zugen von Stecknadeln." 



In the fact that the pore- and osculum-bearing surfaces are differentiated, 

 T. alba resembles T. {Blemma) grimaldii Topsent (1892), which apparently 

 lacks the striking peculiarity common to T. alba and T. vagabmida 0. 

 Schm., viz. the smooth dermal membrane filled with tangentially arranged 

 tylostyles. 



