88 ELEMENTS OF BIOLOGY 



deep wounds or punctures that are closed oflF from the air without 

 having been thoroughly cleaned are quite likely to develop tetanus, 

 or other anaerobic infection. The free circulation of clean and fil- 

 tered air through such wounds is an important agent in preventing 

 the growth of tetanus bacilli. 



Colonial Protozoa and the Prototypes of Metazoa. It 

 has been said earlier that two sub-kingdoms of both plants and 

 animals are recognized, on the basis of the number of cells that 

 form the individual, the Protophyta and Metaphyta among plants, 

 and the Protozoa and Metazoa among animals. It may be set down 

 as a rule, however, that these distinctions are not absolutely ac- 

 curate. Invariably between closely related groups one finds inter- 

 mediate types, partaking of the characteristics of both. So between 

 single-cell animals and the commonly recognized multicellular 

 forms, there occur a number of types of organisms that are com- 

 monly grouped with the Protozoa, but are made up of cell aggre- 

 gations or colonies, more or less well organized. Also, the distinction 

 between plants and animals is not sharp, and certain forms are 

 intermediate between plants and animals. A common protozoon 

 that illustrates both the intermediate between plant and animal and 

 between unicellular and multicellular is the common fresh water 

 Volvox. Volvox (Fig. 27) is usually classified as a member of the 

 class Mastigophora. An individual consists of a colony of cells, often 

 a thousand or more, forming a hollow sphere. Each cell is equipped 

 with a tuft of two whip-like flagellae, so that the surface of the 

 spherical colony is studded with flagellar. The members of the colony 

 by their metabolism secrete a gelatinous substance which forms the 

 matrix in which the cells are embedded, each cell being connected 

 with adjacent cells by means of bridges of protoplasm. 



While a volvox colony exhibits many animal characters, being 

 highly motile, lacking cellulose, and in general resembling other 

 Mastigophora, the metabolism is holophytic, chlorophyll being pres- 

 ent. Thus in an important character the organism resembles plants. 



