GENERAL ORGANIZATION 269 



or endoplasm. Superficially, it is covered by an exceedingly delicate 

 membrane or periplast, which does not prevent amoeboid movements, 

 formation of pseudopodia, or the ingestion of food particles at any point 

 of the body surface. There is a single nucleus, while one or more basal 

 granules or blepharoplasts, which lie upon the nuclear membrane or 

 free in the cytoplasm, are also present. From each blepharoplast there 

 arises an axoneme, which may be traced to the surface of the body, and 

 thence into a flagellum, which consists of the axoneme covered by a sheath 

 formed by the periplast. In the non-parasitic forms contractile vacuoles 

 are present. Flagellates of this relatively simple type are usually free- 

 swimming organisms, but some of them are able to attach themselves 

 temporarily to objects by a process like a pseudopodium or, with or with- 

 out losing their fiagella, to crawl about like amoebae for a time. The fila- 

 ments and cup-like receptacles mentioned above are formed as secretions 

 from the surface of the body, as also is the gelatinous matrix which binds 

 together the colonial forms. These structures are not actually parts of 

 the organism, and are not to be regarded as modifications of the superficial 

 layer of the cytoplasm. The more highly developed flagellates may be 

 considered to have arisen from the simpler forms by changes in the peri- 

 plast or by the development of internal structures. With a thicker 

 periplast there is still the possibility of change in body form, though this 

 is limited, while the power of ingesting food at any part of the body 

 surface is lost. A definite cytostome is developed, usually near the origin 

 of the fiagella. With further development of the periplast, the body 

 becomes rigid and a definite body shape is acquired. In many cases this 

 thickened rigid periplast is of a high degree of complexity, and may be 

 elaborately marked. The fiagella, when more than one are present, 

 usually arise near together at the anterior end of the b'ody. Sometimes, 

 however, they are spread over a wider area. In certain forms some of the 

 fiagella arise from the anterior end of the body and one or two from the 

 posterior end. In the case of the latter the axonemes may pass directly 

 backwards through the cytoplasm from their respective blepharoplasts 

 {Giardia, Hexamita), or they may pass forwards through the cytoplasm 

 to the anterior end of the body and, turning backwards, pass over the 

 surface of the body to enter the fiagella when they reach the posterior end. 

 When an axoneme passes over the surface of the body, the periplast may 

 be raised into a ridge or membrane (undulating membrane), along the edge 

 of which the axoneme passes (Trichomonas, Trypanosoma). In certain 

 flagellates the periplast at the anterior end of the body becomes raised 

 into a collar or cuff, which surrounds the fiagella (Choanoflagellata, or 

 collared flagellates). 



In addition to the complexities in organization which are the result of 



