112 



SPONGES 



early stages consists of five layers (Figs. 76, 77) : (1) an outer 

 porous skin, the dermal membrane (d.m) ; (2) within this is a space 

 traversed in all directions by strands of tissue, which constitute 

 the subdermal trabecular layer (sd.tr) ; (3) within this is a continuous 

 layer of thimble-shaped flagellated chambers, the blind ends of 

 which are turned towards the dermal surface, and their openings 

 towards the gastral cavity (fl.c) ; (4) internal to the chambers is 

 another space, traversed by the subgastral layer of trabeculae (sg.tr), 

 quite similar in its structure and appearance to the subdermal 



Sd.tr. 



.-flc. 



FIG. 76. 



Longitudinal section of a young specimen of iMnuglnella pupa, O.S., with commencing 

 formation of the oscular area. The spicules are omitted from the drawing, x 35. (After F. E. 

 Schulze.) d.m, dermal membrane ; sd.tr, subdermal trabecular layer ; fl.c, flagellated chamber ; 

 sg.tr, subgastral trabecular layer; g.m, gastral membrane ; G.C, gastral cavity; osc, region of 

 future osculum. 



layer ; (5) and finally, the gastral cavity is limited by a porous 

 gastral membrane (g.m), which recalls in its structure the dermal 

 membrane. Of these five layers, the third comprises the whole 

 gastral layer; the first, second, fourth, and fifth are differentia- 

 tions of the dermal layer. 



The five layers that have been described recur in the same order 

 and with similar characters in the body wall of all Hexactinellids, 

 which exhibit a remarkable uniformity in this respect. The 

 chief modifications that are met with in the canal system are due 



