Evolution 



from that of the Medusa. There is a central cavity communicating 

 with the outside not only by the mouth but also by two canals 

 opening near the upper pole. There are two radial canals, each 

 of which divides into four, the branches of which lead at right 

 angles into other canals, running from pole to pole and blind 

 at both ends. There are two tentacles, as shown, which can be 

 withdrawn into special sacs. At the opposite end from the mouth 

 are sense organs, seemingly of smell and balance respectively. 

 On the outer surface, above each of the longitudinal canals, is a 

 row of small plates bearing cilia. It is by the movement of these 

 cilia, like a multitude of minute oars, that the animal swims 

 a method of locomotion which does not occur in the true Ccelen- 

 terates. An additional feature is the formation, at an early stage 

 of development, of a definite third layer of cells between the 

 outer and the inner. This layer ultimately forms the greater 

 part of the jelly-like mass of the body. 



Regarding the interrelationships of the various types that 

 we have described, and their respective importance with reference 

 to the descent of man, opinions are somewhat divided. Some 

 believe the Ctenophora to have been derived from the Medusa 

 form, but the more probable view seems to be that they have 

 evolved separately from some earlier and more primitive type 

 than any existing Ccelenterate, and that their ancestors have all 

 been free-swimming and ciliated. Now the Ctenophora are con- 

 sidered, on good grounds, to be somewhat nearly akin to the 

 lowest worms, and thus to stand fairly close to the main line of 

 Evolution. If this view be correct, the whole group of existing 

 Ccelenterates forms a side branch of the Evolution tree. This 

 fact, however, does not take away the importance of the group 

 in relation to the theory of the descent of the higher animals, 

 for the Ccelenterates have certainly retained many of the characters 

 which were possessed by the direct ancestors of man, such, for 

 instance, as the simple digestive cavity, the primitive type of body, 

 consisting of two cell layers, the diffuse and elementary nervous 

 system, and the radial arrangement of parts. Moreover, the 

 course of Evolution in the group, leading from the Hydra to the 



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