THE PORIFERA. 107 



canals, or even to local dilatations of these canals — the so- 

 called " ciliated chambers." 



The same relative disproportion of the ectoderm, with the 

 consequent development of passages which traverse the mass 

 of the sponge, and are provided at intervals with ciliated 

 chambers, is found in the silicious sponges, in which the 

 spicula, if they possess any, are formed by a deposit of silex ; 

 and in which, as a rule, the sponge-corpuscles are supported 

 by a more or less complete skeleton of a tough animal sub- 

 stance, termed keratose. 



Halisarca, however, is devoid both of skeleton and spicula, 

 and the minute structure of the curious boring-sponges — the 

 Cllonm — has yet to be elucidated. 



Ilallphysema and Gastrophysema, of Haeckel, appear to 

 be sponges which get no further than the Gastnda condi- 

 tion, and thus form a connecting link between the Sponges 

 and the Hydrozoa. 



The fresh-water sponge (Spongilla) has been studied with 

 extreme care by Lieberkiihn, and the following account, 

 based upon the investigations of that author, is given for the 

 use of the student to whom Spongilla fluvitdis is likely to 

 be the most readily accessible of the sponges. 



The fresh-water sponge grows on the banks of docks, 

 canals, rivers, and on floating timber, in the form of thick 

 incrusting masses, which usually have a green color, and 

 require a constant supply of fresh water for their healthy 

 maintenance. The surface presents irregular conical emi- 

 nences perforated at their summit like small volcanic craters, 

 and from these exhalent funnels, which answer to the oscida 

 of the Ccdcispongice, currents of the water are continually 

 flowing. Careful examination of the surface of the Spongilla 

 between the exhalent craters, shows that it is formed by a 

 delicate membranous expansion, separating which from the 

 deeper substance of the Spongilla are a number of irregular 

 cavities. In some cases, these run into one great water- 

 chamber. The superficial chambers (or chamber) communi- 

 cate with the exterior by pores, which perforate the mem- 

 branous expansion, are similar to those in the outer surface of 

 the ventricular wall of a simple calcareous sponge, and sub- 

 serve the same inhalent function. On their inner face, or 

 floor, the superficial chambers exhibit the apertures of in- 

 numerable canals, which traverse the deep substance of the 

 Spo?igilla in all directions, and, sooner or later, unite into 

 passages which lead directly into the cavities of the exhalent 



