Introductory. 



15 



chambers within the fleshy part of the sponge wall ; to them run the inhalent 

 canals (epirrhysa) from the external surface of the sponge, and from them 

 depart the exhalent canals (aporrhysd) emptying into the paragastral cavity, 

 through apertures termed ontia. All skeletal structures belong to the 

 connective tissue, that is, the median layer. 



Such, in brief, is the essen- 

 tial structure qi the typical 

 sponge, and from this type there 

 are no departures so extreme 

 as to obscure comparison with 

 this standard. In the simplest 

 sponges, as the Ascones, the wall 

 is thin and there are no special 

 chambers where the ciliary cells 

 are aggregated. The entire sur- 

 face of the paragaster, however, 

 is covered with these flagella. 

 It seems highly probable that 

 a similar condition prevailed 

 among the ancient thin-walled 

 hexactinellids, which are here 

 under consideration. Sciiulze 

 states that ^vhile he could not de- 

 termine the existence of flagella 

 in the hexactinellids on account 

 of the smallness of the cells, he 

 believes that they exist. (Chal- 

 lenger Expedition, loo. cit. ]). 2'?).) 



Fixation is an not invariable character. There are, for example, certain 

 ball-shaped lithistid sponges, like the palaeozoic Astylospongia and Hindia, 

 which appea?' to have been imattached and the sport of the water currents. 

 These sponges, too, as well as others which are attached, present a concentnc 

 arrangement of the water canals ^vhich, according to Rauff, have originated 

 from the ramified furrows of the external surface and have eventually become 

 internal hy progressive over-growth of the sponge in concentric layers. 

 Such concentric canals parallel to the periphery of the sponge, are simply 

 large conduits receiving the external water through a great number of 

 minute, converging epirrhysa and debouching in the small paragaster near 

 the summit of the sponge. 



FiouRE 1. Diagrammatic figure sbowlng the structure of a gimple 

 sponge In vertical section. The sponge substance {fi)\s lightly shaded. 

 The canal system is black, the arrows showing the direction of the 

 currents, pi), are the pores or openings of the inhalent canals 

 (epirrhysa) which lead to the flagellate chambers, ch. From the latter 

 depart the exhalent canals (aporrh.vsa), ic, which open Into the 

 general central cavity or paragaster. o, Is the osculum or aperture 

 through which the water Is discharged. (Haxckel. ) 



