COMPLICATED FORMS OF SPONGES. 



119 



sheath of mesogloea subdermal 



FIG. 33. Diagram showing types of 

 Canal System. (After KORSCHELT and 

 HEIDER.) The flagellate regions are 

 dark throughout, the mesogloea is dot- 

 ted, the arrows show the direction of 

 the currents. All the figures represent 

 cross sections through the wall. 



A. Simple A scon type, EC. ectoderm, En. 



endoderm, Mg. mesogloea. 



B. Sycon type, with flagellate radial 



chambers (r.c.). 



C. Leucon type, with flagellate side aisles 



^on the main radial chambers. 



D. Still more complex type, with small 



flagellate chambers,/; ch. 



chambers, by flat epithelium with 



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 complications are 

 common. 



(d} The ectoderm is 

 usually described as a 

 covering layer of flat 

 epithelium, but flask 

 shaped cells have also 

 been observed (Bidder). 

 It may be folded in- 

 wards, 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 con- 

 tractility, as around the 

 osculum of Ascetta cla- 

 thrus, though the con- 

 tractile elements usu- 

 ally belong to the meso- 

 glrea. 



The endoderm con- 

 sists typically of collared 

 flagellate cells, but in the 

 more complex sponges 

 these are replaced, ex- 

 cept in the flagellate 

 or without flagella. 



