26 THE ANIMAL PARASITES OF MAN 



of the food. To this distribution of functions between the various layers ot cyto- 

 plasm is due the development of particular cellular organs, such as the appearance 

 of cilia, flagella, suctorial tubules (in the Suctoria) and the myophan striations, which 

 are contractile parts of the ectoplasm in Infusoria and Gregarines. In many cases 

 (Flagellata, Ciliata), an area is differentiated for the ingestion of food (oral part r 

 cytostome) to which there is often added a straight or curved opening (cytopharynx), 

 through which the food reaches the endoplasm. The indigestible residue is either cast 

 off through the oral part or excreted by a special anal part (cytopyge). In rare 

 cases, structures sensitive to light, the so-called pigment or eye spots are developed, 

 e.g., Euglena. In the case of Infusoria the endoplasm circulates slowly, and 

 agglomerations of fluids (food vacuoles) sometimes appear around each bolus of 

 food ; in these vacuoles the food is digested under the action of certain materials 

 (ferments). Even in the lowliest Protozoa fluids to be excreted are, as a rule, 

 gathered into one, or, more rarely, several contractile vacuoles, which regularly 

 discharge their contents. This action, however, is to a certain extent governed 

 by the temperature of the surrounding medium. In some Infusoria a tube-like 

 channel in the cytoplasm is joined to the contractile vacuole which usually occupies 

 a certain position ; this forms a sort of excretory duct, and there are also supply- 

 canals leading to these organellae. 



Very frequently various substances are deposited in the endoplasm, such as fatty 

 granules, drops of oil, pigment granules, bubbles of gas or crystals. More solid 

 skeletal substances are secreted in or on the ectoplasm. To the latter belong the 

 cuticle of the Sporozoa and Infusoria, the chalky shells containing one or several 

 chambers of the Foraminifera, the siliceous and very ornamental framework of the 

 Radiolaria, and the chitinous coat of many Flagellata, Infusoria, etc. Some forms 

 make use of foreign bodies found in their surroundings, such as grains of sand, to 

 construct their protective coverings. 



The food often consists of small animal or vegetable organisms and of organic 

 waste ; it is usually introduced in toto into the endoplasm. On the other hand, the 

 Suctoria extract nourishment from their prey by means of their tentacles. Many- 

 parasitic species also ingest solid food, others feed by endosmosis. 



In all cases one nucleus at least is present. It is true that the existence of non- 

 nucleated Protozoa, the so-called Monera, is still insisted upon, but some of these 

 have already proved to be nucleated, and the presence of nuclei in the others will no- 

 doubt be established. Very often the number of nuclei increases considerably, but 

 these multinucleate stages are always preceded by tininucleate stages. In the 

 Infusoria, in addition to the larger or principal nucleus (macronucleus) there is 

 usually a smaller reproductive nucleus (micronucleus). This dualism of the nuclear 

 apparatus is considered by some to be general, and usually to appear first at the 

 onset of reproduction. 



The form and structure of the nucleus vary greatly in different species. There 

 are elongate, kidney-shaped, or even branched nuclei as well as spherical or oval 

 ones. In addition to vesicular nuclei with a distinct karyosome and incidentally 

 also with a nuclear membrane, homogeneous and more solid formations are 

 frequently encountered. The nuclei are always differentiated from the protoplasm 

 by their reactions, particularly in regard to certain stains. 



In many Protozoa an extra-nuclear mass, sometimes compact, sometimes diffuse, 

 arises from or near the nucleus. This mass, whose staining reactions resemble those 

 of the nucleus, is termed the chromidial apparatus. On the dualistic hypothesis, two 

 varieties of chromidia occur, one originating from the vegetative nucleus (macro- 

 nucleus), being chromidia in the restricted sense, the other derived from the 

 reproductive or micronucleus being termed sporetia. Chromidia consist of altered 

 (? katabolic) nuclear material. 



