u6 



PORIFEKA SPONGES. 



fact that the endoderm cells are better nourished and 

 multiply more rapidly than those of the ectoderm, which 



thus fails to keep pace 

 with the inner layer.] 



(c) By infoldings of the 

 skin — ectoderm and a 

 subjacent sheath of meso- 

 glcea — subdermal spaces 

 may be formed ; an outer 

 cortex may be distinctly 

 differentiated from the 

 internal region in which 

 the flagellate chambers 

 occur ; the pores may 

 collect into sieve - like 

 areas, which open into 

 dome-like cavities ; these 

 and many other com- 

 plications are common. 



(d) ■ The ectoderm is 

 usually described as a 

 covering layer of flat 

 epithjelium, but flask- 

 shaped cells have also 

 been' observed (Bidder). 

 It may be folded inwards, 

 as we have noticed, and, 

 according to some, it 

 also lines the incurrent 

 or afferent canals in 

 whole or in part. In a 

 few cases, e.g. Oscarella 

 lobularis, it is ciliated, and 

 its cells may also exhibit 

 contractility, as around 

 the osculum of Ascetta 

 clathrus, though the con- 

 tractile elements usually 

 belong to the mesoglcea. 



Fig. 50. — Diagram showing types of 

 canal system. — After Korschelt and 

 Heider. The flagellate regions are 

 dark throughout, the mesoglcea is 

 dotted, the arrows show the direction 

 of the currents. All the figures re- 

 present cross-sections through thewall. 



A. Simple Ascon type (Jzc, ectoderm ; En., 

 endoderm; Mg., mesoglcea). 



B. Sycon type, with flagellate radial cham- 

 bers (r.c). 



C. Leucon type, with flagellate side aisles 

 on the main radial chambers. 



D. Still more complex type, with small 

 flagellate chambers (f. ck.). 



The endoderm consists typically of collared flagellate cells, 

 but in the more complex sponges these are replaced, except 



