ZOOLOGY A^^) BOTAN-Yj inCEOSCOPY, ETC. 753 



tliose wLicIi are assigned to the wall and body of the cell, and similarly 

 from those of the other parts. 



At this point we cannot insist too strongly on this other fact, that, 

 from the condition of a unicellular organism with independent 

 existence, each of these cells has already produced, owing to the 

 actions of assimilation and dis-assimilation which go on in it, parts 

 exterior to itself, namely, tegumentary and skeletal structures — 

 organs which are not cellular, although they present morphological 

 arrangements often of a very complicated nature. 



We see, then, that fi-om the unicellular state animals produce 

 skeletal parts of comparative complexity, homologous with those 

 which are, in the pluricellular forms, of ectodermal origin. These 

 parts, without being cellular, assume, as early as in the Sertularians, 

 great importance with regard to the constitution and maintenance of 

 the system, while above the Worms, Mollusca and Articulata (in 

 many of which they may form the most considerable part of the 

 organism), they find as homologues only the scales of tish, the teeth, 

 the proper walls of the notochord, the crystalline lens and the 

 glands, the shells and capsules of ova, and some other parts which 

 agree with them in not being cellular, in spite of the frequently great 

 complexity of their intimate or structural arrangements. 



Further, it may be noticed even in the ciliated Infusoria, that 

 special non-cellular structures of different kinds appear, which may 

 or may not be tegumentary, but are the homologues of the ectodermic 

 structures of the multicellular organisms. Such a structui-e is the 

 curious and complicated organ of corneous, or rather chitinous 

 anpearance, shaped like a toothed wheel, which TricJiodina possesses. 

 The animal in this case is represented by a single cell, and is already 

 producing non-cellular organs as complex, characteristic, and im- 

 portant as their analogues which have the form of hooks, &c., and which 

 are observed in the multicellular parasite of O^hryodendron, and in 

 numerous Turbellaria, Trematodes, Cestodes, &c. Tne most important 

 of these organs are those of skeletal character. 



In point of fact these are real units, considered both anatomically 

 and physiologically. The doctrine of anatomical elements, as divided 

 into cellular and non-cellular, takes account of them ; not so the cell 

 theory, which looks at nothing but what is either a true cell or takes 

 the form of rnonera or jplastids ; but that is immaterial. 



The importance of this subject arises from the fact that the 

 skeletal structures above noticed, impart habitually to the Protozoa 

 a semblance of being multicellular or at least bicellular organisms. 

 This is the case especially with all those numeroiis furms among the 

 ciliated, flagellated, cilio-flagellate and Acinetine Infusoria which are 

 provided with a shell or theca. with or without a peduncle. 



In the PodopJinjce and Acinetce more particularly, it is not only 

 the theca which takes morphologically the aspect of a cell or cell-wall, 

 without sharing its structure. In Pvdvjjhri/a, the pedimcle is the 

 part which gives a bicellular appearance to the perfect animal : 

 regarding the body as a whole as a cell, and as forming by itself a 

 single cell in the anatomical and physiological meaning of the word, 



