E. MAESHALLT. FLAGELLATED CHAMBER. 135 



(' mesoderm '), which fact causes the excessive thinness of the 

 trabecuhc and the direct exposure of nearly the entire external 

 surface of the chambers to the proportionally widened incur- 

 rent lacunœ. I will return to this point again when I come to 

 speak of the trabecule. Here let it be remarked that the poly- 

 prosopylar chambers of certain non-Hexactinellids seem to pre- 

 pare the way for the condition seen in the Hexactinellida, and 

 that an opposite departure in the structural respect under consi- 

 deration is found in those sponges which have diplodal chambers 

 and which are usually remarkably compact and fleshy on 

 account of the voluminous development of the parenchyme. 



The beams of the reticular membrane are generally flat and 

 narrow bands of variable width. Here and there, they are very 

 thin and thread-like (see figs. 37, 38). The nodal thickenings 

 are formed by the convergence and juncture of the beams, 

 generally four, but sometimes three or five, in number at each 

 node. They usually contain each a single nucleus (rarely two 

 nuclei lying side by side) and therefore represent the central 

 portion of the choanocyte body, of which the reticular beams are 

 but lateral processes. Sometimes in its course the process is seen 

 to give off a branch or branches, which go to unite either with an 

 adjacent beam or a node. When the membrane is looked at en 

 face, the nucleus appears circular and is surrounded by a proto- 

 plasmic area, which is drawn out into the lateral processes. In 

 profile view or optical section, the nucleus presents an elongate 

 elliptical shape, indicating its marked compression on the plane of 

 the reticular membrane. (See figs. 40, 41, 43). The node itself 

 being likewise compressed, there is scarcely visible a protoplasmic 

 layer on either surface of the nucleus, though at the poles of its 

 elongated axis there exists a small protoplasmic accumulation 



