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 suhdermal trabecular latjcr (sd.tr) ; (3) within this is a continuous 

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

 which are turned towards tlie dermal surface, and their oj)eniiigs 

 towards the gastral cavity (/.<■) ; (4) internal to the chambers is 

 another space, traversed by the suhi/a.'<fr(d layer of trabeculae {sg.tr), 

 quite similar in its structure and appearance to the subdermal 



Fio. 76. 



Longitudinal SPCtion of a young specimen of I/inugineUa pu)xi, O.S., with commencing 

 furmation of the oscular area. The spicules are omitted from the drawing. x35. (After F. K. 

 Schulze.) d.m, dermal membrane ; stl.tr, subilermal trabecular layer ; /.c, flagellated chamlxT ; 

 sg.tr, subgastral trabecular layer ; g.m, gastral membrane ; (>'.(', gastral cavity ; osc, region of 

 future osculuni. 



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

 gastral mcmjn-auc (g.m), whicli recalls in its structure the dermal 

 membrane. Of these five layers, the tliird comprises tlie whole 

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

 tions of the dermal la3'er. 



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

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

 which e.vhibit a n-markable uniformity in this re.sj)ect. The 

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



