32 THE SPONGES 



slightly larger terminal opening called the osculum sur- 

 rounded by what looks like a delicate fringe. If our specimen has 

 been recently collected, and is therefore still in a vigorous con- 

 dition, we shall be able to witness the same interesting sight as 

 described by Professor Grant, particularly if we add a very small 

 pinch of carmine powder to the sea-water in the vessel contain- 

 ing the Sponge. We shall then see how the very fine particles of 

 the carmine powder appear to be drawn towards the sides of the 

 branches, and to pass into the interior of the Sponge through the 

 minute inhalent pores, only to be ejected by an outward current 

 issuing from each large terminal opening, demonstrating that there 

 is some motive power within the Sponge that produces an indraw- 

 ing current of water through the minute inhalent pores (ostia) 

 and an exhalent expelling current from the terminal openings 

 (oscula). 



How these incoming and outgoing currents are produced we 

 can only discover by the aid of the dissecting knife and micro- 

 scope. If, therefore, we cut longitudinally through one of the 

 branches of the Sycon Sponge, using a sharp dissecting knife so 

 as to cleave it cleanly from top to base, we shall see that the 

 terminal opening (osculum) leads into a central passage called 

 " the exhalent canal," which runs the entire length of the cylinder 

 or branch, and joins the passages belonging to the other branches, 

 where they unite at the base, so that there is a regular system of 

 communication throughout the Sponge. Under the microscope 

 we shall be able to see that the walls of these cavities are per- 

 forated by numerous fine apertures, called " the radial" or " fla- 

 gellate canals." We shall also be able to make out that these 

 flagellate canals run side by side with the incurrent canals, and 

 actually communicate with them at certain points by small 

 openings. 



We shall now see that the incurrent canals end blindly at their 

 inner extremities, and do not, as we might have supposed, com- 

 municate directly with the large exhalent canal ; on the other 

 hand, the flagellate or radial canals, while communicating with this 

 central cavity, have no opening at the exterior, their outer ends 

 terminating a little below the outer surface of the branch. There- 

 fore the inflowing current of water, carrying with it particles of 

 organic matter and small living organisms, enters through tl 





