34 THE PROTOZOA 



A. GENERAL MORPHOLOGY 



As might be expected from the wide distribution of the Protozoa 

 and their varied modes of life, each of the several classes contains 

 organisms of varying forms and grades of complexity. In fact, no 

 one form is characteristic of any group, but in all cases where the 

 body is plastic and subjected to an even pressure the form is spher- 

 ical (homaxonic), readily changing, however, into an elongate or 

 monaxonic condition. In the higher types, especially those which 

 are inclosed in a firm membrane, the form is usually asymmetrical, 

 and cannot be interpreted as the direct result of mechanical condi- 

 tions. The homaxonic type prevails among Heliozoa, Radiolaria, and 

 intra-cellular Sporozoa (Coccidiida), and occurs in the simpler types of 

 all classes. The monaxonic form prevails among the Mastigophora 

 and the lumen-dwelling Sporozoa (Gregarinida), while asymmetrical 

 forms are dominant among the Infusoria. In all classes, when for 

 any reason the surroundings become unsuitable, or at times as a pre- 

 liminary to some methods of reproduction, the organisms secrete a 

 thick and resistant protective coating or cyst which is usually 

 homaxonic. 



The various adaptations found in the Protozoa are confined almost 

 entirely to the outer protoplasm or ectoplasm, the inner portion or 

 endoplasm remaining approximately similar in structure throughout 

 the group. The ectoplasm, being in direct contact with the sur- 

 rounding medium, becomes hardened into ectoplasmic coatings of 

 various kinds, serving as protective coverings for the inner endo- 

 plasm. It also becomes differentiated into various external organs 

 of locomotion, of food-getting, of defence and offence, and, in the 

 higher types, into organs of sensation. 



I. The Endoplasm. 



Examined under the low powers of the microscope, the body of a 

 protozoon appears to be made up of a gelatinous, diaphanous sub- 

 stance which, under certain conditions, breaks out of the confines of 

 the cell-membrane, forming irregular globular masses in the water. 

 This phenomenon was early recognized, and under the term " difrlu- 

 ence" was regarded by Dujardin as a special property of sarcode. 1 



Examined under higher powers of the microscope (e.g. with a one- 



1 " Sarcode. Je propose de nommer ainsi ce que d'autres observateurs ont appele une 

 gelee vivante, cette substance glutineuse diaphane; insoluble dans 1'eau, se contractant en 

 masses globuleuses, s'attachant aux aiguilles de dissection et se laissant etirer comme du 

 mucus, enfin se trouvant dans tous les animaux inferieurs interposee aux autres elemens de 

 structure." DUJARDIN, '35, p. 367. 



