8 HlCKSON, Animal Symmetry. 



symmetrical ; but in the struggle for existence between 

 these sedentary forms of animal life in rock-pools, coral 

 reefs and many other favoured localities at the sea bottom, 

 there is so much overcrowding that unless the colonies 

 are able in their growth to overcome the difficulties of 

 their environment by some modification of a definite 

 symmetrical plan of growth they would be starved and 

 stifled by their competitors. Overcrowding, however, is 

 not the only difficulty that the sedentary colonies have to 

 contend with in the struggle for existence at the bottom 

 of the sea. In localities where the population is not very 

 dense there may be the dangers associated with the 

 silting up of sand or mud, with intense wave action in 

 exceptional storms, with special or peculiar set of the' 

 currents and countless other varieties of conditions that 

 are met with at the bottom of the sea. The sedentary 

 colonies have become adapted to these various conditions 

 by strengthening their structures with deposits of calcium 

 carbonate, as in the Corals, by provision for extreme 

 flexibility as in many of the Hydroid zoophytes, alcyonaria 

 and Polyzoa, and by various modifications of their, archi- 

 tecture in adaptation to the complex forces that play 

 upon them. 



But with all these varieties of form and structure of 

 the colony as a whole the general result has been to bring 

 the individual polyps of which the colony is composed 

 into such a position that their food supply may be ex- 

 pected from any direction, and the radial symmetry of 

 the individual is maintained. 



If we take such examples as Madrepora or Millepora 

 among the Corals we find in each genus a very great 

 variety of growth, there are forms with profuse delicate 

 branches and with thick sparse branches, plate like forms, 

 cup-shaped forms, encrusting forms, etc., but the polyps 



