5 6 ORGANIZATION OF THE INFUSORIA. 



examples it is nevertheless a matter of no difficulty to recognize the vital 

 independence and essentially unicellular significance of each zooid produced 

 by the binary subdivision of its predecessor. The entire colony stock, in 

 either of the above-named or cognate cases, represents, in fact, the sum total 

 of the process of segmentation of an original single cell. The only instances 

 in which, so far as is at present known, there would appear to be a complete 

 obliteration of the boundary lines that normally separate each unicellular 

 element or vital area, is encountered in that singular type Dendrosonia 

 radians belonging to the group of the Tentaculifera. In this we find 

 a common repent stolon throwing up numerous trunks, which give rise 

 to lateral branchlets that terminate each in a fascicle of suctorial tentacles 

 similar to those borne by an ordinary Acineta. It can scarcely be main- 

 tained that we have here a simply unicellular organism ; each tentaculiferous 

 branchlet is without doubt the equivalent of a typical Podophrya, or Acineta 

 that has arisen from the longitudinal subdivision of a preceding zooid, and 

 with which it has remained intimately and indissolubly connected. Through 

 the process of imperfectly separated terminal gemmation, a somewhat 

 parallel compound body is produced in the allied genus OpJuyodendron, 

 and more particularly in the new form here described under the name of 

 Ophryodendron inulticapitata. The Podophrya genwiipara of Cienkowski 

 exhibits temporarily, during its characteristic reproductive state, a com- 

 pound condition closely identical with that presented by the normal phase 

 of the last-named species. In each of these last-named instances the 

 derivation of the colony stock from a single primary cell or plastid, is self- 

 evident, and notwithstanding the obliteration, or rather non-development, 

 in the case of Dendrosonia, of all boundary lines between the individual 

 zooids, their essential unicellular significance remains conspicuous. 



Internal and External Differentiation. 



Cnticnlar Eleme?its or Ectoplasm. 



The infusorial body in its simplest type of development exhibits a 

 structural composition substantially corresponding with that of the lowest 

 organized tissue cell or plastid, as defined in a previous page. There is no 

 distinct bounding membrane or cell-wall, and no means of discriminating 

 between the soft, semifluid constituents of the interior and exterior 

 regions ; it is throughout, and apart from the nucleus or endoplast, one con- 

 tinuous mass of granular, but otherwise homogeneous and undifferentiated 

 protoplasm. The greater portion of the members of the several orders of 

 the Pantostomata must be referred to this category. In the next step of 

 advance, the outer or peripheral border of the protoplasmic mass, while not 

 assuming the character of a distinct cell-wall or so-called cuticle, presents, 

 as compared with the inner substance of that mass, a slightly more solid 

 type of composition. The somewhat denser external layer may in this 

 instance be conveniently denominated the " ectoplasm " and the softer inner 



