94 ELEMENTARY ZOOLOGY 



The mouth may open directly into the interior of the 

 which interior may be called the digestive cavity 

 may lead into a simple short tube produced by t 

 vagination or bending in of the body-wall, which m 

 looked on as the simplest kind of oesophagus, 

 oesophageal tube opens into the body-cavity or dige^u\/e 

 cavity. This cavity may be incompletely divided by 

 longitudinal partitions which project from the sides in'.o 

 the cavity. 



The jellyfish or medusoid body-form corresponds^ *,. 

 general to an umbrella or bell. Around the edge of this 

 umbrella are disposed numerous threads or tentacles 

 (corresponding to the circlet of tentacles in the polyp). 

 The mouth-opening is at the end of a longer or shorter 

 projection which hangs down from the middle of the 

 under side of the umbrella. The interior body-cavity or 

 digestive cavity extends out into the umbrella-shaped 

 part of the body, usually in the condition of canals radiat- 

 ing from the centre and a connecting canal running 

 around the margin of the umbrella. 



Structure. Although the Ccelenterata show little in- 

 dication of the complex composition of the body out of 

 organs, as it exists^ among the higher animals, yet they 

 do show an nr|rnfcfokribLe- advance on the simple, almost 

 organless body of the sponges. This is chiefly shown by 

 the differentiation among the cells which compose the 

 body. In the polyps and jellyfishes some of the cells are 

 specialized to be nnrnjgfal'aKlp muscle-cells, some to be 

 \ nerve-cells and fibres, and so on. A very simple nervous 

 system consisting of small groups of nerve-cells connected 

 by nerve-fibres exists. Some very simple special sense- 

 organs may occur. The digestive system, although in 

 the simpler Coelenterates consisting merely of the cylin- 

 drical body-cavity enclosed by the body- wall and opening 

 by the single hole at the free end of the body, in some is 



