128 I. IJIMA : HEXACTINELLIDA. I. 



the external surface of the most peripherally situated flagellated 

 chamhers ; a large portion of it, however, passes, in order to 

 reach the more deeply situated chamhers, into the intertrahecular 

 lacunœ between the chamber-layer evaginations, at places directly 

 and at other places by means of the excurrent canals. As will 

 be shown in the next chapter, the water enters into each 

 flagellated chamber through innumerable minute prosopyles to 

 find exit by a single large apopyle into the intertrahecular 

 lacunœ inside the chamber-layer. It then finds its way out into 

 the gastral cavity either directly through the gastral membrane 

 or by w^ay of the excurrent canals after passing through the canalar 

 membrane, according as the chambers are situated in the deepest 

 part of the choanosome or in more peripheral positions. Final 

 discharge takes places through the oscula in the sieve-plate, 

 in the lateral wall and in the bottom-plate, — probably most ener- 

 getically through those of the first named structure (sieve-plate 

 meshes). 



I will now proceed to give the results of m}^ observations 

 on the structural details and relations of the flagellated chambers, 

 of the trabeculœ, &c., beginning with the 



Flagellated chamber. — Tn shape the individual cham-. 

 hers are generally cup-like, thimble-like or glove-finger-like 

 (see PI. IV, fig. 28)." They mostly measure 80-200//- in length, 



*This, as is well known, is the most usual form of the Hexactinellidan chamber. Quite 

 a different development may however be attained by tlie ohambers of certain Hyalonematid 

 species, tliough in all probahility as the result 'of the secondary branching and anastomosing 

 of the original saccular form. For instance, in Jlyalonema aßine Marsh, and Sericolophus 

 reflcxus (Ij.) {= Hynhinema rejlexum I J.) I have determined after a careful study tiiat the 

 chambers are represented by an intercommunicating system of canals, whose wall consists of 

 the membrana reticularis. Tlie general arrangi'inont of the flagellated canals strongly 

 reminds one of the configuration of a Fan-ca colony. 



