THE MEANING OF EVOLUTION 249 



a spacious body cavity. The Molluscs are unsegmented 

 animals. The dorsal part of their bodies contains the 

 viscera, and is protected by a shell ; while the ventral 

 part is modified for the purpose of locomotion. A fold 

 of integument hangs down all round the body and 

 encloses a cavity in which the gills are contained. The 

 Arthropods are segmented animals. The body is 

 armed by a calcareous carapace or shell which forms 

 the exo-skeleton. Each bodily segment bears a pair 

 of jointed appendages, and also contains a separate 

 nerve-centre. The whole series of ganglia are connected 

 together by means of a nerve-cord, and the nervous 

 system lies ventral to the alimentary canal. The 

 Vertebrata are also segmented animals, but the seg- 

 mentation is not apparent externally. The skeleton is 

 an internal one, and is built up round an axial rod or 

 notochord. The nervous system is situated dorsally 

 to the alimentary canal. There are two pairs of limbs. 

 Thus we set up an essential or schematic structure 

 characteristic of each phylum. These schemata have 

 no real existence : they are morphological types from 

 which the actual bodily structure of the animals in 

 each phylum may be deduced. They represent the 

 minimum of parts which must be present in order that 

 an animal may be placed in the phylum to which we 

 assume that it may belong. But these anatomical parts 

 need not actually be present in the fully developed organ- 

 ism : thus there are Crustacea in which the body is not 

 segmented, and in which neither calcareous exo-skeleton 

 nor jointed appendages are present ; and there are Verte- 

 brata in which the limbs may be absent. But in such 

 cases we require evidence that the essential anatomical 

 characters which are absent in the fully developed 

 animal have appeared at some stage in its ontogeny, 

 and this evidence is usually available. Or if embryo- 



