Unicellular and Multicellular Animals 



it is to receive and transmit stimuli. They have long fibrous 



projections connecting them with each other, so that there is a 



network of communication throughout the whole animal. In 



the Medusa, where co-ordinated movements of various portions 



is necessary, there is a concentration of nerve cells into a double 



ring near the edge. 



Here also there are 



special organs, prob- z"""^. -/A<v s. 9 



ably of sight and of 



the sense of balance ; 



but as these cannot be 



regarded as the fore- ^ y| 



runners of the analo- '\J } 



gous organs in higher 



animals, we need not 



pause to describe them. 



The anatomy of the 



Coelenterates will be 



better understood if 



the reader will study 



the diagrams in Figs. 



35 and 36, while some 



idea of the beauty and 



variety met with in the 



group may be obtained 



from Fig. 37. 



There is another 

 group of jellyfish-like 

 marine animals which 

 have been given the name of Ctenophora. By some they are 

 regarded as a divergent sub-class of the Coelenterates, by others 

 as a distinct main group ; in any case they appear to be important 

 from our point of view. The structure of a typical member is 

 shown in Fig. 38, and a few other forms are illustrated in Fig. 39. 

 Our typical example is pear-shaped, with the mouth at the lower 

 pole. The internal cavity is complex, but is on a different plan 



53 



ram of Cterophore. 



/, Tentacle ; /s, tentacle sac ; /, central cavity ; 

 /cr, upper canal ; ri/d, plate bearing cilia ; C"? radial 

 canal ; r, longitudinal canal ; s/, sense organ. 



