THE NEW EVOLUTION 



Sponges, therefore, cannot develop any very definite symmetry, 

 or a head end. Having no possibility of developing organs of 

 locomotion or for the prehension of prey they must remain 

 throughout their lives attached and are forced to find their 

 support through the development of an efficient mechanism for 

 straining minute organisms from the water. No other type of 

 existence is possible for them. When viewed at right angles to 

 the plane of attachment sponges are always more or less circu- 

 lar — or rather irregularly circular. 



The symmetry of the coelenterates is always a modification of 

 a hemisphere — a sphere with one side pushed in forming a double- 

 walled hemisphere. The coelenterates are therefore radially 

 symmetrical. This radial symmetry renders them incapable of 

 effective locomotion in any definite line, since their sense organs 

 and nervous system are arranged in a circle lying in a plane at 

 right angles to the central axis and at some distance from it — 

 that is, about the periphery of the open pole of the hemisphere. 

 But this circular arrangement of the sense organs and nerves 

 rendering every sector of the animal as alert and as efficient as 

 every other sector peculiarly fits the coelenterates for remaining 

 fixed in one place and reaching out and capturing the organisms 

 in the water about them. Most of them feed upon other animals 

 of considerable size. They are by far the most successful and 

 the most numerous of the fixed animals. They are also fairly 

 successful as free-swimming animals. Not a few of them are 

 very large. Some of the attached gorgonians reach a height of 

 fifteen feet, and one of the common jellyfishes of northern seas 

 reaches a width of seven and a half feet across the bell, from 

 which depend tentacles more than one hundred and twenty feet 

 in length. 



The ctenophores are commonly regarded as representing a sec- 

 tion of the Coelenterata. They were separated from the Coelente- 

 rata as a distinct phylum by the present author in 192.1. They 

 differ markedly from the coelenterates in various significant ways, 

 especially in their symmetry, in the presence of mesoderm, in the 

 entire absence of asexual reproduction and therefore of colony 

 formation, and in the absence of stinging cells. The ctenophores 

 have two body axes, one long and one short, crossing each other 

 in the middle at right angles. The two halves of the body on 

 either side of these two axes are alike. The body is thus divided 



[2.72.] 



