202 



PORIFERA 



occurs also in some adult Hexactinellids, e.g. in Walteria of the 

 Pacific Ocean (Fig. 90). Thus is represented in this order the 

 second type of canal system described among Calcarea. More 

 frequently, however, instead of forming a smooth sheet, the 

 chamber-layer grows out into a number of tubular diverticula, 

 the cavities of which are excurrent canals ; these determine a 

 corresponding number of incurrent canals which lie between 

 them. In this way there arises a canal system resembling 

 the third type of Calcarea. By still further pouching so as 

 to give secondary diverticula, opening into the first, a com- 

 plicated canal system is formed, as, for example, in Euplectella 

 suberea. 



To return to the skeleton, the most complete fusion is attained 

 by the deposit of a continuous sheath of silica round the apposed 

 parallel rays of neighbouring spicules. This may be termed the 

 dictyonine type of union, for it occurs in all those forms originally 

 included under the term Dictyonina, in which the cement 



deposited pari passu with tin 

 formation of the spicule 

 In other cases connectin 

 bridges of silica unite th 

 spicules, or there may be 

 connecting reticulum 

 siliceous threads, or, agai 

 rays crossing obliquely ma 

 be soldered together at the point of contact. These mo 

 irregular methods occur in species where the spicules are free 

 their first formation. Spicules originally free may later 

 united in a true Dictyonine fashion. The terms Lyssacina an' 

 Dictyonina are useful to denote respectively : the former a 

 those Hexactinellida in which the spicules are free at their firs 

 formation, and the latter those in which the deposit of th 

 cementing layer goes hand in hand with the formation of th 

 spicules. But the terms do not indicate separateness of origin 

 of the groups denoted by them, for there is evidence tha 

 Dictyonine types have been derived repeatedly from Lyssacin 

 types, and that in fact every Dictyonine was once a Lyssacine. 



The real or natural cleft in the class lies between those gener* 

 possessing amphidiscs (Figs. 94, 97) among their microscleres, an( 

 all the remainder of the Hexactinellida which bear hexasters (Fig 



Fig. 94. Amphidisc, at a are traces of the 

 four missing rays. 





