128 



SPONGES 



be distinguished : first, an external or enveloping portion, contain- 

 ing no chambers, termed the ectosome ; and secondly, an internal 

 portion, containing the chambers, termed the choanosome. The 

 former is a new acquisition ; the latter constitutes the whole body 

 in such a form as Plakina monolopha or in the Rhagon. 



In correspondence with these changes the incurrent canal 

 system can now be distinguished territorially, so to speak, into 

 two portions, the one lying in the ectosome, the other in the 

 choanosome. Each portion of the incurrent canal system may 

 exhibit very various modifications in different forms, as the result 

 of different modes of growth on the part of the ectosome. Simple 



Fio. 86. 



Vertical section of Stellctta phrissens, Soil. Young specimen, showing the choanosome folded 

 within the cortex, o, oaculuiu. (After Sollas, "Challenger" Report, x50.) 



instances of the two extreme types of the incurrent system, 

 connected, nevertheless, by numerous transitions, are furnished by the 

 genus Tetilla on the one hand, and by some species of the genus Plakina 

 on the other. In Tetilla (Fig. 85) the water on passing through the 

 dermal pores enters wide sinuses lying in the ectosome immediately 

 beneath the dermal membrane, and these spaces can be distinguished 

 as subdermal cavities from the narrower portions of the incurrent 

 canals which traverse the choanosome. The distinction between the 

 ectosomal and choanosomal portions of the incurrent system is still 

 better seen in such a form as Stelletta phrissens (Fig. 86), where the 

 incurrent canals proper are more narrowed, and contrast with the 

 wider subdermal cavities of the ectosome. 



