72 
T H E TASMA N IAN N AT U R AI AST. 
As the Protozoa are almost entirely quite minute and require a 
microscope we will leave them and pass to the next Phylum, the Porifera 
(pore bearing animals) or sponges. It must be borne in mind that the 
ordinary bath sponge is not an entire sponge but merely the silky 
skeleton of one. Living sponges all agree in having a means of com¬ 
pelling a currant of water to enter into their walls by means of a number 
of pores, the water current then traverses a series of canals in the wall 
of the sponge and finally leaves by a series of larger pores. Ry means 
of this current of water small animals are taken into the body of the 
sponge, and in addition the animal obtains the oxygen necessary for 
its existence. 
Some young sponges are very simple, consisting of a cylinder 
fastened to a rock at one end and with an opening at the other end. 
The water current passes directly through the wall of the cylinder into 
the internal cavity and out by the opening at the upper end. 
In a sponge found near Hobart (Syeon gelatinosum) the water 
current has to pass through two sets of canals before reaching the in¬ 
ternal cavity as shown in Figs. 2, 3 and 4. In the ordinary bath sponge,, 
when alive, the canal system is very much complicated. 
Fig 3.—A cylinder of Sycon , cut to show the internal cavity and the direction of the- 
water current. Some of the canals are shown. 
Fig. 4. —The canals in the wall of Sycon much enlarged. The water enters the canal' 
(in) from the exterior, then passes through pores in the wall by the 
canal («:.), then to the cavity within the sponge, and thence to the ex¬ 
terior again. 
Sponges have a peculiar supporting skeleton, consisting sometimes- 
of fibres, sometimes of small bodies called spicules made of silica or 
lime. Sometimes the skeleton is made up of a mixture of fibres and 
spicules. According to the character of the skeleton sponges may be 
said to be calcareous, silicious or horny. The spicules are often of very 
egular shape as seen in Fig. 5. The skeleton of some silicious sponges. 
