INTRODUCTION 



33 



upon the alveolar make-up of the protoplasm, the interalveolar 

 deposits of silica, etc., taking the form of spicules as in Heliozoa and 

 many Radiolaria, of bars, hexagons, rings, fenestrated capsules, 

 etc. (Fig. 11). 



Fig. 11. 



-Schematic figure illustrating the modifications of skeletons according to 

 mechanical principles of deposition. (After Dreyer.) 



(8) Mode of Life.— A third factor determining form is the mode of 

 life. As we have seen floating forms are usually homaxonic or 

 spherical; freely moving types on the other hand are usually mon- 

 axonic. The type form of a freely moving flagellate or holotrichous 

 ciliate is ellipsoidal, the cell being drawn out with its main axis 

 extending in the direction of movement. Attached forms are usu- 

 ally polyaxonic or radially symmetrical, the variations in form 

 depending upon the nature of the attaching portion. Some for 

 example are attached by the protoplasm of the posterior end of a 

 cylindrical body {e. g., Cotlmrnia, VaginicoUa, etc.); others by the 

 more or less stalk-like attenuated end of the body (e. g., Scyphidia, 

 Podophrya, etc.) ; and others by chitinous stalks of variable length 

 (Vorticella species) which may be more or less branched (Dinobryon 

 species, Episiylis, Carchesium, Zoothamnmm, etc.). In the same 

 individual the form may change with change in mode of life, well 

 illustrated by Dimorpha mvtans (Fig. 12), by Nagleria gntheri or 

 Trim astiga m oeha. 



Methods of food-getting and the nature of the food are also potent 

 factors in determining form. ]\Iany of the diatom- and desmid-eating 

 ciliates, whose food lies on the bottom, are characteristically flat- 

 tened forms with the mouth on the under, or physiological ventral, 

 surface (holotrichous ciliates belonging to the genera ChUodon, 

 Orthodon, OpistJwdon, Chlamydodon, Loxophylhnn, etc., and the 

 majority of the h\'potrichous ciliates). Special food-getting, or 

 current-directing, organs frequently modify the form as in the 

 collared flagellates (Choanoflagellates) and in t.\^es like Folliculina 

 ampulla, Bursaria truncatcUa, cephalont gregarines, Pleuronema, 

 etc. Shifting of the position of the mouth in response to different 

 3 



