E. MAESHALLI. — TEABECULiE. 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 liexactinellid 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 Anlage 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- 

 laj^er, becomes lacunose, the lacunai breaking through on the 

 external surface, we should have the internal and the external 

 trabecular system. An interruption in the chamber-layer puts 

 the lacunœ 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 lacuuœ 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- 

 Eeport, PI. LIII, fig. ö. Thus, it is not difficult to derive the 

 structural plan of Hexactiuellids, peculiar though it is in several 



*In view of the apparent presence of cliambers before the imniigration of the external 

 flagellated cell-layer in some Monaxonid larvae, and of tlie results arrived at by R. Evans 

 (Quart. Jour. Micr. Sc, n. s., vol. 42, pt. 4) in Spongilla, the possibility of certain other cells, 

 which have always lain in the internal cell-mass and which have preserved the blastomeric 

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



