STRUCTURE 31 



Each cilium arises from a tiny deep-staining dot or basal granule 

 (Fig. 2G), which, however, is probably not homologous with the 

 blepharoplast of the flagellates. 



Various modifications of the organelles of locomotion occur, 

 e.g., the undulating membrane of many flagellates (Fig. 2D), 

 formed by a delicate membrane connecting a flagellum with the 

 body; the " collar" of the choanoflagellates (Fig. 2E); the mem- 

 branelles and cirri of ciliates (Fig. 2F and H), formed by the 

 fusion of rows or groups of cilia; and the axopodia (Fig. 2B) of 

 some Sarcodina formed by the development of supporting rods 

 in pseudopodia, thus making a permanent structure. Of quite 

 a different nature, but none the less organelles of movement, are 

 the myonemes (Fig. 1, myo.), found in many Protozoa, and cor- 

 responding to the muscle fibers of Metazoa. They enable the 

 animals to twist and bend their bodies. The myonemes are 

 extremely delicate contractile fibers which run in various direc- 

 tions in the ectoplasm of the animal; they occur most commonly 

 in flagellates and ciliates. In some protozoans structures have 

 been described which show every evidence of being highly or- 

 ganized neuromotor apparatus, i.e., a definitely arranged and 

 organized substance having a nervous control over the contrac- 

 tile fibers or myonemes (Fig. 1, mot.). 



Organelles for food-taking occur chiefly in the flagellates and 

 ciliates. Such protozoans may have a " cytostome " or cell 

 mouth for the ingestion of food (Fig. 1), and a " cytopyge " or 

 cell anus for the elimination of waste matter. They may also 

 have a delicate membranous pharynx (Fig. 1, ph.) for leading 

 the food material into the endoplasm, and food vacuoles (Fig. 

 1, f.v.) into which the food is accumulated and in which it is 

 circulated inside the body. In some protozoans, namely the 

 Suctoria, a much modified group of ciliates, there are developed 

 sucking tentacles for the absorption of food. In others there 

 are tiny capsules in the ectoplasm containing minute threads 

 which can be shot forth when stimulated, and used either to 

 overpower prey or for protection from enemies. For the ex- 

 cretion of waste products of the body there is often present one 

 or more contractile vacuoles (Fig. 1, c.v.), little cavities in the 

 protoplasm of the body which expand with water containing 

 urea and other waste matters conducted to them by tiny radiating 

 canals, and which periodically contract , forcing their contents 



