692 COELENTERATAI. PORIFERA. 



chambers which may be called "whip chambers." The water, entering 

 through the pores on the outer surface, passes through canals into these 

 spherical chambers, the whips of the lining cells keeping it in movement 



and sending it on through 

 The Canal System, other canals, which unite 

 with those from other 

 whip chambers to form large channels that 

 finally open into the central cavity. When 

 such a sponge is cut across and highly 

 magnified we see, as in the diagram Fig. 2, 

 an elaborate and repeatedly branched system 

 of canals. The whip chambers unite the 

 ramifications, which come from the outer 

 surface (the incurrent canals), with those 

 that lead to large trunks opening into the 

 gastral cavity (the excurrent canals). F{ ^_- DlknM OP TIIE CAKAL 



Ihe above descriptions apply only to what SYSTEM OF A SPONGE. 



maybe regarded as single sponge individuals ; 



but, by repeated budding, individual sponges may, in some cases, form colonies 

 in which the individuals are no longer separately recognisable. This bud- 

 ding process giving rise to colonies is, however, not the 

 Development of a only method of reproduction. In a Sponge whose life- 

 Sponge, history has been fully worked out, the process is as 

 follows : Eggs are produced at certain parts of the body- 

 wall which, when fertilised, divide up and give rise to small oval embryos 

 covered with cilia. These are either solid or are provided with a small 

 central cavity. They escape from the parent through the oscule, and swim 

 about for a short time, varying from a few hours to a day or two. During 

 this time they change their shape, and spicules develop in their walls. 

 When such an embryo attaches itself and becomes a sponge, the whip cells 

 which clothed the outer surface and rowed the tiny creature about, come, in 

 various ways, to line the interior and set up currents of water which bring 

 in the necessary food. 



Although the canal system is so essential to the life of the Sponges, and is 

 so typically developed in the different kinds, its various modifications are 

 apparently of little use in classification. Those who have 

 Classification of made Sponges a special study prefer to group them accord- 

 Sponges, ing to the structure of the hard parts which support the 

 outer layer of the body. These hard parts are either 

 spicules or fibres, and are very rarely absent. 



In the Calcareous Sponges, the skeleton consists of separate spicules of 

 carbonate of lime. In the Siliceous Sponges, flinty spicules are found, which 

 either remain separate or become united together to form networks or other 

 supporting structures. There are, further, Sponges with skeletons composed 

 of horny fibres, and besides the spicules or fibres secreted by the Sponge 

 itself, foreign objects are sometimes appropriated to assist in the formation 

 of the skeleton. 



The spicules found in Sponges assume a vast variety of shapes, a few of 

 which are given in Fig. 3. Some are like fine needles pointed at both ends, 

 others have several rays, the number of these determining the classification 

 of the Siliceous Sponges into those with six or four-rayed or uniaxial spicules. 

 Other spicules, again, terminate either at one end or both in anchor-like 



