ENTOMOLOGY FOR MEDICAL 

 OFFICERS 



CHAPTER I 



Introduction 



Although this book does not purport going much beyond 

 the Arthropoda with which the medical officer is concerned, 

 yet there are certain broad facts regarding the classification 

 and geographical distribution of animals, the characters and 

 relations of the Arthropod Phylum, and the general bearing 

 of Arthropoda on human pathology, that have to be con- 

 sidered very briefly by way of introduction. 



I.— Classification of Animals. 



Animals are arranged in two primary series. The one 

 series— Protozoa — includes animals that consist either of a 

 single cell or of a small aggregate of cells that show little or 

 no diversity ; the other series — Metazoa — includes animals 

 that begin life as a single cell, which gradually develops into 

 an organised community of diversified cells. 



The Metazoa again are arranged in two series. The 

 one series consists of animals like the Sponges (Porifera), 

 which are composed of groups of cells scattered without any 

 definite bond of union in an overwhelming mass of inapt 

 intercellular substance ; the other series — Enterozoa — con- 

 sists of animals that have a body-wall composed of definite 

 and adjusted elements, and — certain degraded parasites being 

 left out of account — a mouth and a well-defined digestive- 

 cavity. 



A 



