Evolution 



anemones, jelly*fishes, corals, etc. At first sight their structure 

 seems vastly different to that of the Volvox, from some form 

 similar to which they have probably been derived. The evi- 

 dence obtained from the study of their individual development, 

 however, strongly suggests a process by which we suppose that 

 they evolved from Volvox-like ancestors. We shall therefore 

 briefly describe the earlier stages of the development of a coral. 

 The sexually produced individual starts life as a single cell, the 

 fertilised egg. This divides and redivides until a hollow ball 

 of cells is produced, which cells, like those of the Volvox, bear 

 cilia. Although simply spherical in shape, the creature moves by 

 rotating round a definite axis, like a planet. Moreover, nutri- 

 ment is absorbed not by any or every part of the surface, but 

 only by a small area round the lower pole. Now as development 

 proceeds, the cells at this pole divide more rapidly than the rest, 

 with the natural result that the ball begins to get out of shape. 

 The distended portion, however, develops to the inside, so that 

 one part of the sphere is, as it were, pushed into the other. 

 When this process has been completed, the original internal 

 cavity is almost entirely eliminated, and a form is produced which 

 resembles a double-walled flask or vase. Such a form may be 

 taken as the fundamental architectural type of the groups that 

 we are now to consider. The meaning of this further step of 

 Evolution is again specialisation. The inner layer of cells takes 

 on the functions of digestion and absorption of food, there having 

 been evolved, in fact, the simplest possible form of mouth and 

 stomach. Such other functions as those of locomotion, protection, 

 and support are exercised by the outer layer. This process is 

 illustrated in Fig. 31. 



But there is no known type of animal which, in its adult form, 

 shows quite the simple structure that we have described. Per- 

 haps the nearest approach is to be found in the lower sponges, in 

 which two modifications of the original plan have already been 

 introduced. In the first place, the creature is sedentary, being 

 fixed, in an inverted position, to some solid basis. It has, so to 



speak, ceased to be a hunter, and is become a fisher. Secondly, 



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