EUrLECTELLA. 41 



tare with the parietal ledge and must therefore be regarded as 

 such under special development. 



The main mass of the cuff" and parietal ledges, as in fact 

 all parts of the lateral body- wall except the principal framework 

 of the skeleton, consists of loose tissues (Flockengewebe, flake- 

 tissues), which easily fall off on rough handling or can be 

 pulverized by rubbing between the fingers. 



The internal or gastral surface of the body- wall (PL II, fig. 

 5 ; PL IV, fig. 4) shoAvs low and narrow ridges, mainly circular 

 and longitudinal, brought about by the underlying principal 

 skeletal beams (PL II, fig. 9), which are situated much nearer 

 to this than to the external surface. Many of the quadrate 

 meshes formed by the ridges contain each a depression, the 

 bottom of which is perforated by a parietal osculura. Other 

 meshes, the so-called ' interstitial ' meshes, inclose one or more 

 apertures of the larger excurrent canals, while smaller excurrent 

 canals open everywhere on the ridges as well as on the surface 

 of the perforated depressions. 



Anatomically and so far as the soft parts are concerned, I 

 consider the walls of all Hexactinellids as being composed of 

 three layers (PI. IV, fig. 28 ; PL V. fig. 36). These from the 

 exterior inwards are successively : 1) The external trahecular layer, 

 which corresponds in part with the ectosome of Sollas. 2) The 

 complexly evaginated layer of chambers, constituting the essen- 

 tial portion of the choanosome. 3) The internal trabecular layer, 

 which may partly develop into an endosome. The first and the 

 third are histologically the same and may in that sense be 

 united into one, so that we distinguish in a general way only 

 two kinds of differentiated tissues, the flagellated chamber-wall 

 (membrana reticularis) and the trabeculse. What F. E. Schulze 



