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- 

 ably of sight and of 

 the sense of balance ; 

 but as these cannot be 

 regarded as the fore- 

 runners of the analo- '* 

 gous organs in higher 

 animals, we need not 

 pause to describe them. 

 The anatomy of the 

 Ccelenterates will be 

 better understood if 

 the reader will study 

 the diagrams in Figs. 



35 and 36, while some \* "1 HUH'**/ 



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 



rud 



FIG. 38. Diagram of Cterophore. 



Tentacle ; _/>-, tentacle sac ; /, central cavity ; 

 tg, upper canal ; rud, plate bearing cilia ; g, radial 

 canal ; r, longitudinal canal ; si, sense organ. 



