10 PRIMITIVE ANIMALS 



Arthropod. This question will be considered in more 

 detail in a future chapter. 



Now, although the discovery that all these various 

 organisms could be shown to be parts of one intricate 

 evolutionary process from a common type was a 

 satisfactory achievement, yet the origin of the Ap- 

 pendiculate phylum, as indeed of every other phylum, 

 remains shrouded in mystery, save for the possible 

 glimpse of light thrown upon it by the organization 

 of the Nemertea. The great and characteristic ac- 

 quirements of the Appendiculate phylum, by which 

 it shows an advance of organization over all the 

 lower phyla, are as follows, though we must bear in 

 mind that most of them may be lost in particular 

 cases as the result of degeneration. Firstly, as the 

 name implies, the Appendiculata possess limbs, 

 muscular processes of the body which may become 

 jointed and complicated in structure. Secondly, their 

 bodies are built up of a head and a tail and an inter- 

 vening region of a varying number of similar rings or 

 segments following one another in linear series. Each 

 one of these segments may carry a complete set of 

 organs, a pair of limbs, a nerve-ganglion, a pair of 

 excretory organs and so forth, so that, discounting 

 the head and the tail, the organism may be said to 

 be built up by the serial repetition of a number of 

 homologous and similar parts. Such an organism, 

 e.g. an Earthworm, is said to be metamerically 



