lyo 



PORIFERA 



exposure and consequent suspended activity. The oscula then 

 serve for the exit of currents of water carrying particles of solid 

 matter, while the entrance of water is effected through the ostia. 

 Sections show that the ostia lead into spaces below the thin 

 superficial layer or " dermal membrane " ; these are continued 

 down into the deeper parts of the sponge as the " incurrent 

 canals," irregular winding passages of lumen continually dimin- 

 ishing as they descend. They all sooner or later open by numerous 

 small pores " prosopyles " into certain subspherical sacs termed 

 flagellated chambers. Each chamber discharges by one wide 

 aperture " apopyle " -into an " excurrent canal." This latter is 



Fig. 64. H. panicea : the arrows indicate the direction of the current, which is made 

 visible by coloured particles. (After Grant.) 



only distinguishable from an incurrent canal by the difference in 

 its mode of communication with the chambers. 



The excurrent canals convey to the osculum the water which 

 has passed through the ostia and chambers. All the peri- 

 pheral parts of the sponge from which chambers are absent are 

 termed the " ectosome," while the chamber-bearing regions are the 

 " choanosome." 



The peculiar crumb-of- bread consistency is due to the nature 

 of the skeleton, which is formed of irregular bundles and strands 

 of minute needles or spicules composed of silica hydrate, a 

 substance familiar to us in another form as opal : they are 

 clear and transparent like glass. They are scattered through 

 the tissues in great abundance. 



The classes of cellular elements in the sponge are as follows : 

 Flattened cells termed " pinacocytes " cover all the free surfaces, 

 that is to say, the external surface and the walls of the excur- 



I 



