120 I. IJIJIA : HEXACTINELLIDA. I. 



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

 canalar himon disappears or rather becomes indistinguishable 

 from the ordinary intertrabecular lacunae ; 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 wliich per- 

 vades the external recesses between the evaginated protuberances 

 of the chamber-layer. They branch during their inward coui'se 

 and may nndergo 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 lacunae and cavities 

 among the trabeculae 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 subdermal stratum ; although it 

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

 well-defnied demarcation between them. The ectosomal stratum 

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

 skeleton and is more or less specialized in consequence of that 

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

 stratum is characterized by its relatively more spacious lacuna? 



