COMPLICATED FORMS OF SPONGES. 



119 



sheath of meso.aflcea — subdermal 







Fig. 33. — Diagram showing types of 

 Canal System. (After Korschici.t and 

 Heider.) The flagellate regions are 

 (lark throughout, the mesoglcea is dot- 

 ted, the arrows show the direction of 

 the currents. All the figures represent 

 cross sections through the wall. 



A. Simple Ascon type, Ec. ectoderm, Efi. 



endoderm, .1/^. mesoglcea. 



B. Sycoft type, with tlagellate radial 



chambers i^r.c.'). 



C. Lencon 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 

 chaiTibers 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- 

 thriis, though the con- 

 tractile elements usu- 

 ally belong to the meso- 

 glcea. 



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. 



