i STRUCTURE AND PHYSIOLOGY OF ANIMALS 43 



ment of internal parts. Such an animal is said to be radially 

 symmetrical. The body of a radially symmetrical animal is capable 

 of being divided into a series of equal radial parts or antimeres, 

 each of which is symmetrically disposed with regard to one of the 

 secondary or radial axes. 



In animals which are not permanently fixed, locomotion usually 

 takes place in the direction of the primary axis of the body, and 

 one side, habitually directed downwards, becomes modified differ- 

 ently from the other, which is habitually directed upwards : a lower 

 or ventral surface becomes distinguishable from an upper or dorsal. 

 Thus the radial symmetry is now disturbed ; the secondary axes 

 have become unequal ; the dorso-ventral or vertical secondary axes 

 are, to a greater or less extent, different from the transverse or 

 horizontal secondary axes, and the body of an animal having such 

 a disposition of the parts is divisible into two equal lateral halves 

 or hemisomes by a median vertical plane passing through the 

 primary axis. This is the bilateral symmetry observable in all but 

 a few types of animals. 



Sometimes the bilaterally symmetrical animal is unsegmented ; 

 sometimes it is divided into a series of segments or metameres. 

 A distinct head may be present or absent. The head end or 

 anterior end is that which, save in exceptional cases, is directed 

 forwards in locomotion. It is towards this end that the organs of 

 special sense are situated, as well as the opening of the mouth and 

 the organs for the prehension and mastication of food. A head is 

 developed when the anterior part bearing these structures is 

 marked off externally from the rest. In segmented animals the 

 head consists of a number of segments amalgamated together, and 

 it contains the brain or the principal central ganglia of the nervous 

 system. 



8. THE PRIMARY SUBDIVISIONS OR PHYLA OF THE ANIMAL 



KINGDOM. 



The various systems of organs digestive, circulatory, nervous, 

 excretory, etc. present under one form or another in all the higher 

 groups of animals, are variously arranged and occupy various 

 relative positions in different cases, producing a number of widely 

 different plans of animal structure. According as their structure 

 conforms to one or another of these great plans, animals are referred 

 to one or another of the corresponding great divisions or phyla of 

 the animal kingdom. That animals do present widely differing 

 plans of structure is a matter of common knowledge. We have 

 only to compare the true Fish, such as Cod, Haddock, etc., in a fish- 

 monger's shop with the Lobsters and the Oysters, to recognise the 

 general nature of such a distinction. The first-named are charac- 

 terised by the possession of a backbone and skull, with a brain and 



