. Natural history. Zoology. 692 COELENTERATA—I. PORIFERA. chambers which may be called "whip ; 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 channel's 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
. Natural history. Zoology. 692 COELENTERATA—I. PORIFERA. chambers which may be called "whip ; 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 channel's 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). ^^^ of the Canal The above descriptions apply only to what System of a Spokoe. 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. AlthouL'h the canal system is so essential to the life of t
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