694 



COELENTERATA—I. PORIFERA—IL CNWARIA. 



iris-like membrane ; fine pores are to be fuund opening on the surface 

 between the conical prominences. Canal systems with whip chambers run 

 through the entire mass, all the cavities and canals being lined with flat cells, 

 while the collar cells are restricted to the whip chambers. The tissue 

 through which the canal systems run receives firmness from the presence 

 of the horny fibres, which are specially plentiful along the chief canals and 

 round the oscules. 



The Bath Sponge, like most of the other massive Sponges, may be regarded 

 as a colony, each individual of which consists of a cavity opening through an 

 oscule and of the canal system which finally leads into that cavity. Such 

 colonies arise by budding ; the reproduction of the colony is, however, 

 eifected by means of eggs which groiv into free-swimraing larvaj. 



The Bath Sponge flourishes on the coasts of the eastern half of the 

 Mediterranean, in the West Indies, the Bahamas, and Florida. The 

 sponges are fished for either with h-irpoons or nets, or are obtained by means 

 of divers, and are carefully prepared for the market. Large Sponges are also 

 artificially produced from small pieces which, under favourable conditions, 

 grow to a marketable size in the ^pace of about seven years. 



II. — The Cnidaria. 



The Cnidaria, which form the other great division of the Ooelenterata 

 derive their name from the peculiar stinging cells with which they are 

 provided. They have been divided into : — (1) The Hydrozoa, or water 

 animals, to which all the Zoophytes or plant-like animals, and some of the 

 Jelly-fish belong. (2) The Scyphozoa, which include most of the larger 

 Jelly - fish, the Sea - anemones, and the 

 Corals. (3) The Ctenophura or Comb- ^^^^ 



bearers. 



A short account of one of the simplest 

 and most easily accessible of the Hydrozoa 

 will serve to give some idea of the essential 

 features of the Cnidarian body. The Hydra 

 is a small fresh-water Hydrozoan very 

 common in ponds and 



The Hydrozoa. aquaria. As it is from 

 J to ^ an inch in length, it 

 can easily be seen with the naked eye hang- 

 ing from the under surface of some water 

 plant or other submerged object, to which 

 it has attached itself by a very simple kind 

 of sucker at the lower end of the body. 

 The free end terminates in a mouth at the 

 tip of a prominence which is surrounded 

 by a ring of long tentacles. Both the body 

 and the tentacles are exceedingly contrac- 

 tile, so that while the whole animal, when 

 retracted, appears like a gelatinous ball 

 crowned with bud-like knobs, when fully 

 extended its body is slender and the 

 tentacles thread-like. 



The walls of the simple tube-like body 

 of the Hydra consist of two layers of cells- 



Fig. 6. 



TIIS FRBSH-WiTBR POLYP ( Fydrn). 



Stinging Cells (Nematocysts), much 

 magnified. 



A, The vesicle with thread discharged. 



B, The vesicle containing coiled thread. 



-an outer layer in contact with the 



