The Development of the Animal World 71 



evolution." The corals are a higher development on the 

 same lines. They are found in co-operative communities 

 in the Silurian period. The hydrozoa and medusae are 

 another line of very great interest. All these — and the 

 medusae — are essentially simple animals with mouth- 

 opening and primitive stomach, but they have now 

 developed rudimentary nerve and muscle cells and 

 definite sex cells, as well as tentacles and streamers and 

 stinging organs. A good idea of the typical structure 

 can be obtained by flattening a Hydra — found in most 

 ponds — under a moderate power (say one-eighth) of the 

 microscope. Each cell in its body stands out with 

 wonderful distinctness, if it is carefully placed. 



So far the line of evolution is fairly traceable, but we 

 now come to a point where it is very obscure. From 

 these sessile or fixed animals none of the higher types 

 have been developed. We have to turn to the other 

 alternative — the animals that swam about in the water 

 in search of food. Broadly speaking, it is clear that this 

 habit would lead in time to the formation of a worm-like 

 and ultimately fish-like organism; but the development 

 in detail is difficult to follow. Consider the evolution of 

 the boat. At first a clumsy tree-trunk hollowed out by 

 fire, it has come, from the need of greater speed, to have 

 a long, evenly-balanced (or bilateral) body, with a 

 definite head and tail (or stem and stern). Its eye 

 (look-out) is at the front, its excreta trail behind, its 

 heart and heavier organs (machinery, etc.) are safely 

 encased in the centre. Undoubtedly the same principles 

 controlled the evolution of the fish, through a worm-like 

 stage.* 



* Note carefully that I do not say "worm." I am thinking 

 of a " water- worm " with a straight body and cilia, not of our 

 misleading "earth-worm." The word "worm" is in fact now 

 almost abandoned in zoology. In its time it covered a multi- 

 tude of sins — in classification. 



