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 Ewplectella 

 suherea. 



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 tlie 

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

 included under the term Dictyonina, in which the cement is 



deposited ^j«?'i passu with tiie 

 formation of the spicules. 

 In other cases connecting 

 bridges of silica unite the 

 spicules, or there may be a 

 connecting reticulum of 



Fin. 94.— Amphidisc, at « are traces of the siliceOUS threads, Or, again, 



four missmg rays. ^^^.^ crossing obliquely may 



be soldered together at the point of contact. These more 

 irregular methods occur in species where the spicules are free at 

 their first formation. Spicules originally free may later be 

 united in a true Dictyonine fashion. The terms Lyssacina and 

 Dictyonina are useful to denote respectively : the former all 

 those Hexactinellida in which the spicules are free at their first 

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

 cementing layer goes hand in hand with the formation of the 

 spicules. But the terms do not indicate separateness of origin 

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

 Dictyonine types have been derived repeatedly from Lyssacine 

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



The real or natural cleft in the class lies between those genera 

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

 all the remainder of the Hexactinellida which bear hexasters (Fig. 



