120 I. IJIMA : HEXACTINELLIDA. I. 



after tliese have diminished in caliber beyond a certain limit, the 

 canalar lumen disappears or rather becomes indistingaishable 

 from the ordinary intertrabecular lacunse ; in these small branches, 

 as in fact in all evaginations of insignificant dimensions, there 

 persists a primitive condition in that the entire internal space 

 is traversed uninterruptedly by the trabeculse. 



The incurrent canals, it scarcely needs to be specially pointed 

 out, are canalar gaps in the outer trabecular system which per- 

 vades the external recesses between the evaginated protuberances 

 of the chamber-layer. They branch during their inward course 

 and may undergo anastomosis with their fellows. They are not 

 always circular in cross -section. In all Hexactinellida they are 

 as a rule smaller but more numerous than the excurrent canals 

 and further unlike these, they never break through externally so 

 as to open directly onto the surface of the sponge-wall. With 

 their outer apertures they join the lacunar spaces in the peri- 

 pheral trabecular layer, and in fact all the lacunse and cavities 

 among the trabeculœ form one intercommunicating system on 

 either side of the chamber-layer. 



In the peripheral or superficial layer of the external trabe- 

 cular system just referred to, and which I have before mentioned 

 as continuously covering over the outer ends of the chamber- 

 layer protuberances, there may be distinguished two strata, the 

 outer ectosomal, and the inner subdcrmal stratum ; although it 

 must not be imagined that there always exists any sort of a 

 well-defined demarcation between them. The ectosomal stratum 

 or the eclosome is the seat of the latticework of the dermal 

 skeleton and is more or less speciulized in consequence of that 

 fact as well as of its most superficial situation. The subdormal 

 stratum is characterized by its relatively more spacious lacuna) 



