E. MAESHALLI. — TKABECULJi;. 163 



cellular elements, is essentially similar to that of Monaxonid 

 sponges. As to its metamorphosis, not a fact has come under 

 my observation and I shall have to be contented with specu- 

 lations, guided by our modern knowledge of sponge embryo- 

 logy- 



After the Hexactinellid larva has attached itself to some 

 foreign object, the flagellated cells of the external layer should 

 be expected to sink somehow into the internal cell-mass and 

 within this to form the Anlaxje of the chambers.'-' We may 

 presume that these are arranged side by side in a sort of layer, 

 the chamber-layer, which surrounds the central portion of the 

 original internal cell-mass and at the same time is itself sur- 

 rounded externally by the peripheral portion of the same. 

 Suppose the cell-mass, both within and without this chamber- 

 liiyer, becomes lacunose, the lacume breaking through on the 

 external surface, we should have the internal and the external 

 trabecular system. An interruption in the chamber-layer puts 

 the lacunjE in the two systems into direct intercommunication, and 

 such a spot may mark the position of a future osculum (see p. 105). 

 If, in addition, the lacunar undergo certain local expansions, — the 

 one such expansion in the center, as the incipient gastral cavity, 

 being the widest, — we should have a young Hexactinellid essentially 

 agreeing in the arrangement of its soft parts with the little 

 Lanuginella pupa figured by F. E. Schulze in the Challenger- 

 E,eport, PI. LIII, fig. .3. Thus, it is not difficult to derive the 

 structural plan of Hexactinellids, peculiar though it is in several 



* In view of tlie apparent presence of cliambers before tlie immigration of the external 

 flagellated cell-layer in some Jlonaxoniil larva>, and of the results arrived at hy R. Evans 

 (Quart. Jour. Micr. Sc., n. s., vol. i-, pt. 4) in Spongilla, the possibility of certain other cells, 

 wliich have always lain in the internal cell-mass and which liave preserved tlie blastoineric 

 character, giving rise to chambers under certain circumstances, may not be excluded. 



