STRUCTURE OF SPONGES 



157 



ponent cells are better nourished and multiply more rapidly 

 than those of the outer layer. 



(c) By infoldings of the outer layer and a subjacent 

 sheath of mesoglcea — 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 complications 

 are common. 



{d) The covering layer usually consists of flat epithelium, 

 but flask-shaped cells have also been observed (Bidder). 

 It may be folded inwards, as we have noticed, and, accord- 

 ing to some, it also lines the inhalant or afferent canals in 



R.C 



Fig. 77. — Diagram of sponge structure. 



R.C., A flagellate chamber into which water passes by an inhalant pore (LP.). O., 

 An osculum, into which the exhalant canals open ; the arrows indicate the 

 directions of the currents. SD.SP., A subdermal cavity or porch, into which 

 the inhalant pores may open. 



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. 



The inner layer consists typically of collared flagellate 

 cells, but in the more complex sponges these are replaced, 

 except in the flagellate chambers, by flat epithelial cells, 

 with or without flagella. 



The mesogloea contains very varied elements, and illus- 

 trates the beginnings of different kinds of tissue. Thus 

 there are migrant amoeboid cells (phagocytes) ; irregular 

 connective tissue cells ; spindle-shaped connective tissue 

 cells, united into fibrous strands ; contractile cells, e.g. 

 those forming a sphincter around the oscula of some forms, 



