52 THE MOST PRIMITIVE METAZOA 



conditions are not often given equal credit for determining and 

 shaping an animal. Thus various morphological conditions will be 

 associated with a pelagic life, with burrowing, with living in sand, 

 with being a very large animal or being a very small animal. All 

 of these tend to alter the morphology of the animal and make it a 

 successful living animal, not just a representative of a hypothetical 

 idea; that of, say, a crustacean. An example of such an environ- 

 mental effect can be seen when we come to consider which is the 

 more primitive, radial symmetry or bilateral symmetry? (Fig. 17.) 



Radial symmetry 



This type of symmetry is often found in sessile or pelagic 

 animals. They usually have an oral and an aboral surface but 

 otherwise any diameter cut at right angles to the oral-aboral axis 

 should divide the animals into tw r o similar halves. In fact most of 

 the animals that are radially symmetrical do not fit in with this 

 definition since they usually have some irregularity in their 

 organisation, i.e. mesenteries, tentacles, madreporite, which allow 

 only certain sections at right angles to the oral— aboral axis to 

 divide the animal into equal halves. 



Bilateral symmetry 



The animals that show bilateral symmetry are organised into 

 an antero-posterior axis and a dorso-ventral axis. In addition there 

 is one plane and one plane only that will separate the animals into 

 equal right and left halves. A radially symmetrical animal will 

 have many such planes. Most of the Metazoa that are not pelagic 

 or sessile show a bilateral symmetry. (Fig. 17.) 



One is often taught that the coelenterates and the echinoderms 

 show a basic radial symmetry and that the other Metazoa are 

 bilaterally symmetrical. Since the echinoderms and coelenterates 

 are sometimes placed at the foot of the metazoan evolutionary 

 tree it is not difficult to associate radial symmetry with a primitive 

 habit and to assume that the bilaterally symmetrical condition is 

 the more advanced. On the other hand, it is equally true that 

 radial symmetry is found in sessile or floating animals whilst 

 bilateral symmetry is found in crawling or swimming animals. 

 We are therefore left with the question, "To what extent does the 

 symmetry of an animal indicate its primitiveness and to what 

 extent does it reflect the habits of that animal? " 



