6 TROPICAL MEDICINE AND HYGIENE 



characters approach the Animal Kingdom and were 

 formerly included in it, others are allied to the fungi 

 and Vegetable Kingdoms, such as the bacteria or schizo- 

 mycetes. Members of all classes cause disease. 



The multicellular organisms definitely animal in 

 character are known as the Metazoa, and this kingdom 

 constitutes the animal kingdom in the restricted sense. 

 These metazoa forming the animal kingdom are divided 

 into sub-kingdoms, the important ones for our purposes 

 amongst the invertebrata being Vermes, Arthropoda and 

 Mollusca. The Vermes, or worms, are represented by 

 the Platy helminths, such as flukes and tapeworms, and 

 the Nematohelminths, as filariae, round-worms, thread- 

 worms, hook-worms, &c. The Arthropoda, animals with 

 jointed limbs, include the classes Insecta, Arachnida 

 (spiders, scorpions, and ticks), Crustacea (including 

 cyclops, the carrier of the Guinea-worm, and possibly of 

 other parasites). The next sub-kingdom or phylum is 

 the Mollusca, some of which, as snails, oysters, &c. r 

 carry parasitic larvae or micro-organisms. 



Of the vertebrate classes, snakes (Ophidia) alone are 

 considered. The higher vertebrates, when used as food, 

 occasionally cause disease, and may carry the larval forms 

 of parasites which may develop in man to the adult stage, 

 The carnivora are directly dangerous to man, but these 

 are too well known to require description. 



Genera and Species. The division into genera and 

 species has given rise to much discussion, and the 

 experts have found it necessary with each increase of 

 knowledge of the subject to modify the classifications. 



As a general rule, division into genera and species is 

 or should be a convenience, and unless the reasons are 

 very definite, genera should not be formed on single 

 species. On the other hand, a genus composed of a very 

 large number of species is difficult to work with, and 

 subdivision of a genus is necessary, either into several 

 genera or sub-genera. 



Species are sometimes divided into sub-species ; this 



