UNICELL ULA R NA TURE. 5 5 



of the entire sum of Protozoic structures, and embraces among the Infusoria 

 proper, as here comprehended, no less than four out of the total of thirteen 

 orders that make up the series. 



Although the unicellular nature of the Infusoria is here fully accepted 

 and maintained, it has yet to be admitted that in a considerable number of 

 instances such unicellularity has become somewhat obscure. This is 

 more especially observable among the order of the Ciliata, where we find 

 several representatives of the Opalinidae possessing an almost indefinite 

 number of nucleus-like structures, a like complexity in this respect being 

 also exhibited by the Trachelophylltim apiculaUun of Claparede and Lach- 

 mann, and a few other Holotricha. Among the Hypotrichous order of the 

 Ciliata, the nucleus is, again, rarely single, being more usually represented 

 in duplicate. Other forms, such as Loxophyllum meleagris may be further 

 quoted as illustrative of examples in which the nucleus exhibits a condition 

 of modification midway between the two previously quoted series. This 

 frequently recurring composite character of the nucleus has been seized 

 upon by those who are unwilling to concede to the Infusoria the nature and 

 position of unicellular structures, as affording substantial evidence in support 

 of their objections. No single cell or unicellular organism, in their opinion, 

 can possess more than a single nucleus, and where there is a multiplicity of 

 such structures there must, they maintain, likewise be multicellularity ! As 

 explained more at length, however, later on, this structure, the nucleus, as 

 encountered among independent Protozoic organisms, presents an amount 

 of variation and complexity not met with in simple tissue cells, exhibit- 

 ing more especially in the present connection a capacity to subdivide 

 within, and independently of its surrounding protoplasm and peripheral 

 cell-wall, where such exists. The distinction in both aspect and properties 

 of the nucleus, as thus viewed, from its normal condition as the single 

 central and essential constituent of a simple tissue cell, is so obvious that 

 doubts have been naturally expressed as to whether the so-called nucleus 

 of the infusorial body can be regarded as the precise equivalent of the 

 structure that takes the same name in the latter instance. It has been 

 further elected by Professor Huxley, in face of these doubts, to confer 

 upon the nucleus, or its seeming equivalent as associated with Protozoic 

 structures, the distinctive title of the " endoplast," and which title is accepted 

 and for the future mainly adopted throughout this treatise. 



The fundamental unicellular structure of Protozoic or infusorial 

 organisms is masked in a yet entirely opposite direction, the obscurity 

 arisino- in this instance from the imperfect separation of the zooids 

 produced through the ordinary process of duplicative division. Familiar 

 examples of this type of modification are afforded by the compound colo- 

 nies of the Flagellata Ajithopliysa and Codosiga, and various Vorticellidae, 

 such as Zoothamnium and Carchesium, in all of which a greater or less 

 number of the divided zooids remain intimately united with one another 

 through the continuity of their supporting pedicles. . In all of these 



