78 PORIFERA. 



over by a membrane (Fig. 67, Z) which is exactly comparable to 

 the dermal cortex of the Calcarea, and is pierced by inhalent pores 

 leading into the incurrent sinuses. (Figs. 67, 68.) This membrane 

 is called the ectosome, as opposed to the rest of the sponge, which 

 contains the chambers and is called the choanosome ; and the 

 incurrent sinuses beneath it constitute the subdermal cavities, which, 

 as is obvious, correspond to the intercanals of the Calcarea. By 

 further folding of the spongophare and suppression of chambers on 

 the main excurrent sinuses an increase in complexity is obtained. 



In the Eurypylous type the chambers open directly into the 

 excurrent sinuses ; but in many sponges they are removed from 

 the surface of the latter, and the apopyle of each of them is pro- 

 longed into a canaliculus the aphodus lined by a prolongation of 

 the epithelium of the excurrent sinus into which it opens. Further, 



FIG. 68. Diagram of a section ot the outer part 01 Tetilla <pedifera (after Sollas, from Perrier). 

 C choanosome ; E ectosome ; i incurrent sinus; v flagellated chamber ; g excurrent sinus. 



there is only one prosopyle to each chamber. This is the Aphodal 

 modification of the Khagon type (Stelletidae, Geodinidae, etc.). 



Finally there is the Diplodal chamber system (Fig. 69), in which 

 there is a canaliculus leading to the prosopyle of each chamber 

 the prosodus as well as an aphodus leading from it (Cho?ulrosina, 

 Corticium candelabrum, etc.). 



In Eurypylous sponges the ectosome never attains any special development ; 

 but in other types it becomes greatly thickened and histologically differentiated, 

 and is called the cortex. In such cases the tubes leading through the cortex 

 from the sieve-pores to the subcortical (subdermal) cavities are called chones, 

 and are provided at some part of their course with a muscular sphincter, the 

 velum. The inhalent pores may be diffuse or collected into pore-areas forming 

 the so-called sieve-plates. 



Histology. There is but little to add to the statements in the 

 general account. The most remarkable feature is the flagellated 

 cells or choanocytes. They are larger in the Calcarea than in most 



