THE SHAPES AND SIZES OF ANIMALS 131 



adult condition are attached either temporarily or 

 permanently. Of these the common fresh-water 

 Hydra, and the whole of the great group of Hydro- 

 zoa, known popularly as zoophytes, are familiar 

 examples. The majority of these attached radial 

 animals reproduce by budding, the buds usually 

 remaining attached to their parent, and so giving 

 rise to plant-like colonies. In many of these, and 

 especially in the higher group of Coelenterates, the 

 Actinozoa, of which sea anemones and corals are 

 instances, a curious modification of the typical 

 radial symmetry is manifested, to which the term 

 biradiate symmetry is commonly applied. In a 

 biradiate animal, while the radial symmetry is well 

 preserved, there is superadded to it a further 

 change in the shape and arrangement of certain of 

 the internal organs, whereby a definite plane of 

 symmetry is established, on either side of which 

 the organs are perfectly similarly and symmetrically 

 arranged, but which is the only plane by which the 

 animal can be so divided. 



The simplest case of biradiate symmetry would 

 be of this kind : Imagine a Hydra with the body, 

 as usual, cylindrical, i.e., circular in transverse 

 section, and with the mouth also circular in outline; 

 and, for the sake of simplicity, imagine the tentacles 

 to be absent. Such an animal has any number of 

 planes of symmetry, for any plane of division 

 passing along the whole length of the animal and 

 along its axis will divide the Hydra into two 

 perfectly symmetrical halves. Now, imagine the 

 mouth of our Hydra to become oval or elliptical in 



