ORDER HYPOTRICHA. 739 



observed by Mr. Slack to become more pointed posteriorly, and to exhibit towards the 

 centre a fusiform dilatation as in the more normal examples examined by the author. 

 Quite recently, October 1881, the author has obtained luxuriant colonies of this 

 species attached to Myriophyllum growing in a pond in Epping Forest, visited in 

 connection with a field-day of the Epping Forest Naturalists' Field Club. It was 

 observed of examples preserved for some few days that the zooids freely abandoned 

 their original mucilaginous zoocytium, and, reattaching themselves independently, 

 were scarcely distinguishable during such isolated condition from those of Gerda 

 fixa, described in a previous page. 



Order IV. HYPOTRICHA, Stein. 



Animalcules free-swimming, mostly flattened or compressed ; loco- 

 motive cilia confined to the inferior or ventral surface, often variously 

 modified; the superior or dorsal surface usually smooth or glabrous, but 

 occasionally bearing a few scattered or longitudinal rows of immotile setose 

 cilia ; oral and anal apertures conspicuously developed, ventrally located ; 

 trichocysts rare. 



Excepting for the very considerable augmentation of the number of genera 

 they contain, the several family groups of the Hypotricha embodied in the present 

 volume correspond substantially with those introduced by Stein in the first and 

 second parts of his ' Organismus,' published respectively in the years 1859 and 1867. 

 The only important modification of that author's scheme is accomplished through 

 the transference to this order from the Holotricha of the genera Loxodes and 

 Litonotus recently shown by Wrzesniowski to be ciliate only on their ventral 

 aspect, and by the" slight alteration that has necessarily been effected in the diag- 

 nosis of the order through the demonstration by Engelmann and other investigators 

 that certain Oxytrichidae bear immotile setose cilia upon their dorsal surface. These 

 dorsal appendages are, however, altogether distinct from those which clothe the 

 ventral aspect, being rigid and immotile and taking no part in either the locomotive 

 or prehensile functions. While in many of the more specialized genera of the 

 Oxytrichidae and Euplotidae the members of the Hypotricha appear to arrive, though 

 by an entirely different path, at an even higher structural organization than is 

 exhibited by any of the preceding groups, the subordination of others as embryonic 

 or more simple types of the order Peritricha is likewise apparent. This is more 

 especially evidenced in such genera as Phascolodon and Chlamydodon, and which, 

 excepting for their dental armature, remarkably resemble the embryonic con- 

 dition of the Peritrichous genus Spirochona. But a slight modification would again 

 be required to convert Psilotricha into a typical representative of the free-swimming 

 Peritrichous family Halteriidae; the earlier condition of Styloplotes appendiculatus 

 exhibits also a closely analogous affinity. The passage of the Hypotricha to 

 or from the Holotricha and Heterotricha is clearly indicated by such genera as 

 Chilodon, Loxodes, and Litonotus, the characteristic dental armature of the whole of 

 the Chlamydodontidae pointing more especially to a no very distant union with the 

 Holotrichous forms Prorodon and Nassula. The more specialized structure of the 

 higher Hypotricha receives full attention in the account given of the family Oxytri- 

 chidae. As typified by this highly differentiated group, the Hypotrichous order occupies 

 the summit of the graduated scale of Ciliate complexity, and may be said, in virtue 

 of the diverse modifications and functions of the appendicular organs, to claim with 

 relation to the preceding sections a position closely corresponding with that held by 

 the Insecta with reference to the lower divisions of the Arthropoda. 



In accordance with our present state of knowledge, the succeeding schedule 

 would appear to supply the simplest key to an intelligent apprehension of the 

 numerous families and genera referable to the order Hypotricha. 



T 2 



