32 SPONGES 



Calcarea, and, as will be shown when these forms are discussed, 

 the Olynthus may undergo various processes of growth and folding 

 of the body wall without departing from this type, of which 

 the essential characteristic is that all the 

 canals and spaces between the pores and 

 the oscular rim are lined by collar cells, 

 and by collar cells only ; in other words, that 

 the gastral layer is continuous (cf. Figs. 65, 66). 

 Second Type of Canal System. This 

 type arises from the Olynthus, first by 

 a process of unequal growth and con- 

 sequent folding of the body wall, result- 

 ing in the formation of a number of 

 blind diverticula of the gastral cavity ; 

 and secondly, by the restriction of the 

 collared epithelium to the interior of the 

 diverticula in question, which are hence 

 Fl - 4S - termed flagellated chambers (Fig. 44, A 



and B). The central portion of the gas- 



tl%al Cavifc y becomes lined b 7 flattened 



direction of the currents, in this epithelium derived from the dermal layer, 



and in the next three figures, the i -i n in 



thick black line represents the and similar in all respects to the flat 



epithelium of the external surface of the 

 body. Between the flagellated chambers, 

 which may vary considerably in form and length, canals are en- 

 closed along which the water flows to enter the chambers. From 

 their mode of origin it can be seen that the lumen of these incurrent 

 canals, as they are termed, is simply a portion of the outer world 

 enclosed between the folds of the body wall, and lined by the flat 

 epithelium of the outer surface ; and further, that the apertures 

 by which the water enters the chambers are nothing more than the 

 pores of the Olynthus. 



At their first formation the diverticula of the body wall are 

 distinct one from another, and may remain so in a few instances, 

 but more often they tend to coalesce where they touch each 

 other, and also, by thickening of their outer or distal extremi- 

 ties, to form a cortex. In this way two sub-grades of the second 

 type can be distinguished. In the first (Fig. 44, A) the incur- 

 rent canals are wide spaces, continuous with one another between 

 the chambers. In the second (Fig. 44, B) the coalescence be- 

 tween the chambers narrows the incurrent spaces to definite 

 canals, which commence by an opening on the outer surface of the 

 cortex. The sponge as a whole now no longer shows a folded 

 surface, but appears simply as if its body wall was greatly 

 thickened, thus reverting in form to the Olynthus type. The water 

 enters the incurrent canals by definite apertures on what is now the 



