4O THE PROTOZOA 



(C 6 H 10 O 6 ), calcium carbonate (CaCO 3 ), and silica. The secretions 

 may take the form of plates, of continuous deposits, or of regular 

 skeletons which are often extremely complex (Fig. 13, D). In the 

 majority of cases, the secretions are made in the ectoplasm, although 

 in one well-authenticated case at least (Etiglypha alveolata, Fig. 13, A), 

 the plates destined to form the shell are formed in the endoplasm and 

 in the immediate vicinity of the nucleus. 1 In other shell-bearing 

 forms of Rhizopoda, there is usually a basis of chitin, upon which 

 the various shell-substances are deposited, or the shell may consist of 

 the chitin alone. In some cases it is no more than a cap covering a 

 small portion of the body, and into which the entire protoplasmic 

 mass could not possibly be withdrawn (Pseudochlamys> Fig. 13, C). 

 Here the chitin which forms the shell is perfectly smooth ; but in 

 other forms it may be ornamented in various ways by pits or pro- 

 tuberances. Again, in many fresh-water Rhizopoda the shell-material 

 is not secreted, but the test is composed of foreign particles, such 

 as diatom shells, sand crystals, mud, or detritus of any kind, fused 

 together and to a chitinous substratum by means of mucilaginous 

 cement secreted by the organism. 



3. Nuclei. 



Haeckel's claim ('68) that there are organisms without nuclei 

 (Monera), although it rests upon negative evidence, cannot be rejected 

 until all of the forms considered have been shown to possess them. 

 On purely a priori grounds, it is possible to conceive such organisms, 

 although the numerous experiments which have been performed dur- 

 ing the last decade upon nucleated and non-nucleated parts of Pro- 

 tozoa, show, in these cases at least, the absolute necessity of the 

 nucleus for the life of the organism. These experiments make it 

 probable that the so-called Monera have in reality some structure or 

 structures which perform the functions of the nucleus, although a 

 well-defined nucleus with membrane and other characteristic parts 

 may be absent. 



In the majority of Protozoa there is but one nucleus (many Sar- 

 codina, Mastigophora, Sporozoa), while in some forms two nuclei are 

 the rule (some Rhizopoda). In others, again, there may be a great 

 number of nuclei, the number varying with the age of the organism 

 (examples occur in all groups of the Protozoa). In many of the Pro- 

 tozoa, although not in all, the nucleus is provided with a membrane 

 and contains two substances ; chroma tin, staining with certain basic 

 dyes and consisting largely of nucleinic acid, and achromatin, a sub- 

 stance which is not stained by the chromatin dyes, in the form of a 



1 CL Schewiakoff ('88). 



