THE CLASSIFICATION OF ANIMALS 33 



nerves or neurons run parallel with one another, forming a 

 bundle which is called a nerve. A single axon enclosed in 

 its special sheath, the origin of which is doubtful, is termed 

 a nerve fibre. The bodies of the neurons are arranged in 

 many cases in clumps which are termed ganglia (Gr. ganglion, 

 a knot), and the bands of nerve fibres connecting them are 

 termed commissures. 



We have already seen that gland cells are in the vast 

 majority of cases modified epithelial cells, and it therefore 

 follows that glandular tissue is merely modified epithelial 

 tissue. In most cases, as we have seen, glandular tissue is 

 arranged in the form of layers of epithelial cells, forming 

 the walls of tubes. A bundle of such tubes surrounded and 

 separated from one another by connective tissue would be a 

 typical example of glandular tissue. Finally, the reproductive 

 cells themselves are very often arranged in the form of tubes, 

 and a mass of such tubes, with their supporting connective 

 tissue, constitutes a reproductive organ, and might be termed 

 reproductive tissue^ 



CHAPTER V 



THE CLASSIFICATION OF ANIMALS. 



IT is common knowledge that there are an enormous number 

 of sorts of animals in the world. Every one knows also that 

 some of these sorts resemble each other closely, whilst others 

 are widely distinct. Thus, for instance, a missel-thrush and 

 a song-thrush are closely similar to one another; whilst there 

 is apparently little or no resemblance between a star-fish and 

 an elephant. When the matter is looked into closely, it is 

 found that it is possible to arrange all animals in groups, 

 the members of which agree in the common structural plan 

 on which their bodies are built. Such groups are termed 

 Phyla, from the Greek word signifying primarily leaf or shoot, 

 and secondarily stock or race. The Protozoa, or animals 

 devoid of cellular structure, constitute one such phylum ; but 

 the Metazoa consist of many phyla. It would take too long 



