28 GENERAL ORGANIZATION OF THE PROTOZOA 



of offence as well as protective organs, and the minute hunters stalk 

 about with them in search of prey (see page 77). 



(^d) Vacuoles. — The other formed structures of the inner proto- 

 zoan body are the vacuoles. These for the most part are mere fluid- 

 filled spaces, but in many cases they possess a definite and permanent 

 form and are frequently complicated in structure. 



The vacuoles are either storage or contractile vacuoles. The former 

 are minute improvised stomachs, and in them the food matters are 

 digested. The latter are the more complex structurally, varying from 

 simple spaces, which fill with fluid and empty to the outside in rhythmic 

 periods, to great branching canal systems with storage reservoirs and 

 contractile vesicles, the excretory system permeating the entire inner 

 protoplasm with a network of vessels. 



(e) Nuclei, Chromatin, and Chromidia. — At the present time no 

 one who has made a careful study of protozoan cells accepts Haeckel's 

 view ('66) that some forms of unicellular animals are without nuclei 

 (Monera). It is, indeed, true that there are many forms in which nuclei, 

 in a morphological sense, are not permanently retained, but the essen- 

 tial part of the morphological nucleus — the chromatin — is invariably 

 present. Sometimes this chromatin is distributed uniformly through- 

 out the cell (the "distributed nucleus" of tetramitus, dileptus, etc.), 

 but usually it is concentrated about a central body (division centre) 

 having some of the attributes of a centrosome, or it is confined within 

 a firm nuclear membrane. 



Within the last four years there has developed an ever-growing 

 tendency to recognize in protozoa two distinct types of nuclei. These 

 are distinguished from one another in the majority of cases not by 

 any structural characteristics, but by their functions in the cell. One 

 type, the trophonucleus, has to do with the ordinary vegetative func- 

 tions of metabolism. The other type, which may be designated the 

 karyogonad, or simply the gonad nucleus, has no function in ordinary 

 metabolism, but is the source of chromatin forming the nuclei of con- 

 jugating gametes. In a broad sense, therefore, the karyogonad repre- 

 sents the germ plasm of protozoa. 



The forms assumed by the chromatin in these two types of nuclei 

 vary within wide limits. In many cases both are included within one 

 common nuclear membrane, and are separated from one another only at 

 periods of maturation in preparation for fertilization (most gregarines, 

 coccidia, and many flagellates). In other cases the gonad nucleus 

 becomes separated from the trophonucleus at an earlier period in the 

 life history of the individual, and appears in the cytoplasm in the form 

 of distributed chromatin granules (idiochromidia of many different 

 genera, "chromidialnetz," etc.) or as compact and homogeneous 

 nuclei (micronuclei of infusoria, "secondary" or gametic nuclei of 

 sarcodina). 



