CLASSIFICATION OF SPONGES. 17 



a fairly long excurrent canal. The fresh-water sponge is organized 

 essentially upon this plan. 



In the Leuconidae, e.y. Leucandra nivea, a further stage, Fig. 7, 

 can be made out, where instead of the excurrent canal of each 

 chamber pouring its tiny stream direct into the paragaster, those of 

 large groups of chambers are collected, as a river on its journey to the 

 sea gathers its tributaries, into a great and wide, but short stream 

 emptying by wide outlet into the central cavity. 



This, the true Leucon type, is what Sollas terms the Aphodal 

 type of Rhagon. A further development, the Diplodal Rhagon, is 

 reached when the incurrent canals, in this highest type usually 

 restricted to one to each chamber, arise not singly and separately, 

 but rather in the manner of the numerous branches into which a 

 great river divides and sub-divides on its way through its delta. 

 A large and wide tubular cavity, the sub-dermal chamber, pene- 

 trates the crust of the sponge, and from this, numerous wide incurrent 

 canals lead inwards, throwing off at intervals still smaller tubules, 

 each of which feeds a ciliated chamber. The excurrent canals remain 

 the same as in the aphodal type. Often the sub-dermal chamber is 

 arched over by a finely perforated membrane, the pore area, a natural 

 filter against the entrance of enemies and coarse particles. A little 

 below this filter membrane (inwards) is usually a well defined sphinc- 

 ter muscle, adapted by its power of contraction to completely or 

 partially close the entrance to the water canals. The portion of 

 the large chamber inward of the sphincter has received the name 

 of Subcortical crypt. 



A moment's consideration will show the great advantage that 

 will accrue to the sponge if the soft ground tissue be laced with 

 a network of intertwining fibres or supported by a cunningly arranged 

 scaffolding of strong and rigid rods. Without some such support, 

 the flagellated chambers would be apt to collapse and perform 

 their function indifferently, if at all. Based upon the nature of this 

 supporting skeleton, whether or not it is composed of calcareous 

 spicules, we get our two first great divisions, the Calcarea and 

 Non-Calcarea. The latter are by some termed Fibrospongiae, because 

 of a lacework of horny fibres more or less developed. But the 

 majority of the Non-Calcarea have, superadded, spicules formed of 

 silica (Silicispongiae), while others again want both spicules and 

 fibrous skeleton (MyxospongiaB). The Silicispongise are split up into 

 three orders according to the shape of the principal spicules 

 (megascleres), thus : 

 Order I. MONAXONIDA, megascleres simple, rod-shaped (Figs, la 



and 2a, PL iii). 



Order II. TETRACTINELLIDA, megascleres four-rayed (Fig. 4fr, PI. iii). 

 Order III. HEXACTINELLIDA, megascleres six-rayed. 



