Manchester Memoirs, Vol. Ix. (1916). 7 



number of body somites are seldom found. It is other- 

 wise with the radially symmetrical animals in which 

 variations from the normal do not in the same way inter- 

 fere with the movements of the body or power of obtain- 

 ing food. 



But to return to the question of the body symmetries. 

 We have seen that a radial symmetry is frequently asso- 

 ciated with the sedentary habit and that this association 

 is probably correlated with the need for collecting food in 

 any direction from which it may approach the animal. 



Another method, however, adopted by many classes 

 of sedentary animals for increasing the food catching 

 powers, is by the formation of colonies by gemmation or 

 fission from the original individual developed from the 



These colonies, as seen in the Corals, Alcyonaria, 

 Hydroid zoophytes, Polyzoa, Tunicata, etc., are frequently 

 supported by skeletal structures of Calcium carbonate, 

 Chitin or Keratin, frequently attain to a great size and 

 assume an infinite variety of shapes! In many cases the 

 shape may be described as definitely radially symmetry. 

 In any well-equipped Museum there may be seen the 

 large dome-shaped masses of brain coral, the long cylin- 

 drical unbranched colonies of Juncella, the bush-like 

 growths of Seriatopora branching profusely in all direc- 

 tions from a common centre, and many other forms that 

 have a more or less complete radial symmetry, but still a 

 very large number of them show a method of growth and 

 branching which is clearly not on a radially symmetrical 

 plan, and the question arises : How can these other shapes 

 be accounted for ? 



A sedentary colony receiving food for its individual 

 members from all directions should be radially sym- 

 metrical, just as a Hydra or a Sea-anemone are radially 



