56 



A. T. MASTERMAN 



[JANUARY 



B. 



A 



B. 



Fig. 2. — I)i-axo-symmetry. 

 (4 A + 2 B.) 



merit already referred to. The conditions for the acquirement of 

 di-axo-symmetry are a horizontal position of the main axis of the 



organism with free rotation about this axis. 



Certain Eadiolaria, such as the Acantharia, 

 ^a. according to Haeckel, present this ground-form, 



and the Amphistomina, certain of the Ciliata, 

 —A, and to some extent the Thaliacea, may serve as 



further examples. 



True axo-symmetry is exhibited by a great 



number of animals, besides nearly all the plant 



kingdom. It is found in many of the Protozoa. 



It is the fundamental type of all the Porifera, 



Coelentera, and Echinoderma. Traces of it are 

 found in the Polyzoa, Cirripedia, sedentary Annelida, and Tunicata. 

 The Cuvierian group of Eadiata depended 

 mainly for its institution upon the presence of 

 this type of symmetry. We have only to add 

 that, in the ontogeny of many of the above, 

 and in that of higher Metazoa, this type is A 

 represented by the gastrula. 



A study of the above phyla will show that 

 this axo - symmetry occurs in precisely the 

 environmental conditions in which, from a 

 theoretical point of view, we should expect to 

 find it. They fall into three groups : — 



B 



A. 



..A. 



A. 



B'. 



Fig. 3. — Axo-symmetry. 

 (4 a + b + b'.) 



1. Sedentary. 



2. Pelagic floating (plankton in restricted sense). 



Q 

 O. 



Free pelagic with axial rotation. 



(1) Sedentary. — It is usually granted that axo-symmetry is 

 correlated with a sedentary existence. This statement is in no way 

 invalidated by the instances of Cirripedia and others in which a very 

 evident bilateral symmetry is still in evidence. That the " cirri " of 

 Balanus tend to present an axial arrangement is readily seen by an 

 examination of the living animal from above ; and that the shells and, 

 again, the stalk of L&pas are axo-symmetric in their arrangement is 

 evident. 



The modification from one type of symmetry to another will be 

 proportional in completeness to the time during which the new environ- 

 ment has been effective, and, in addition, to the degree of differentia- 

 tion to which the organism has already attained. Thus, Amoeba will 

 adapt its form almost immediately to a change of environment, a 

 Coelenterate more slowly, and a Cirripede more slowly still. 



If an axis be drawn from the point of fixation of a sedentary 

 organism at right angles to the plane of fixation, the environment on 

 all sides of this axis will be similar (except in special cases, e.g. 



