SPONGES 127 



be termed incurrent canals, though in reality spaces external to the 

 sponge. A condition quite similar in the main to that seen in 

 Plakina monolopJia, occurs also in Oscarella, which differs only in having 

 both apopyles and prosopyles drawn out into distinct aphodi and 

 prosodi, so that the very simple canal system in this form is of the 

 diplodal type (Schulze). 1 



The further development of the canal system is brought about 

 by processes of growth perfectly similar to those already described 

 in the Calcarea Heterocoela ; namely, on the one hand, by further 

 folding of the spongophare, leading to considerable branching and 

 complication of both the excurrent and incurrent canals ; and, on 

 the other hand, by thickenings of, and fusions between, the outer 

 ends of the diverticula of the spongophare, with the result, first, 

 that the incnrrent spaces become more completely enclosed and 



FIG. 85. 



Diagram of a transverse section through the outer region of Tetilla jn'.dijern. E, ectosome ; 

 C, choanosome ; e, excurrent canal ; i, incurrent canal ; p, ostia. (After Sollas, "Challenger " 

 Reports.) 



narrowed to form definite canals ; and secondly, that a cortical 

 layer is developed on the external surface of the sponge body. 



An instructive stage in the evolution of the incurrent system 

 exhibiting but a slight advance on the state of things found in 

 Plakina monolopha^is seen in the Tetractinellid genus Tetilla (Fig. 85). 

 The dermal layer is greatly thickened at the distal extremity of 

 each diverticulum of the spongophare, and the outer free margin of 

 each such thickening is expanded into a rim or plate which unites 

 with the margins of other and similar thickenings to form a 

 continuous dermal membrane, perfectly comparable in its origin to 

 the pseudoderm often formed in an Ascon colony or the dermal 

 membrane of some Heterocoela. Over each incurrent canal the 

 dermal membrane is perforated by the dermal pores or ostia (stomions, 

 Topsent), while the true pores or prosopyles (chamber pores) are 

 now no longer visible on the surface. In consequence of these 

 advances in organisation, two regions of the sponge body can now 



1 The presence of prosodi in Oscarella is disputed by some authors, and it is 

 possibly a variable character ; cf. p. 49, supra. 



