SECTION I. DISCOSTOMATA-GYMNOZOIDA. 325 



Inhabiting salt and fresh water. Increasing by longitudinal or transverse 

 fission and by eneystment and subdivision of the entire body into sporular 

 elements. 



Section L DISCOSTOMA TA-GYMNOZOIDA. 



Collared monads free-swimming or sedentary, loricate or illoricate^ 

 solitary or colonially associated, never in the latter instance completely 

 immersed or hidden within special chambers in a common gelatinous matrix 

 or cytoblastema, but either entirely naked or with their distal regions 

 freely exposed to the surrounding water. 



The extensive and remarkably beautiful series of Flagellate organisms com- 

 prehended under the present sectional title may be said to represent the fruits of 

 the most recent microscopic research. The immunity from discovery and 

 taxonomy which they have for so long previously enjoyed is undoubtedly due to 

 their excessive minuteness, the largest individual zooid in the entire group not 

 exceeding in length the i-i 000th part of an English inch, while in the majority of 

 instances the much smaller calibre of the i-3oooth" obtains. The rapid progress made 

 by opticians, however, within the last few years, in the production, at a moderate 

 cost, of object-glasses of high magnifying power, has placed in the hands of the 

 histologist the key to an entirely new organic world, and of which the present 

 group furnishes, perhaps, a not altogether inappropriate illustration. 



The earliest intelligible record given concerning the existence of the seiies of 

 minute organisms here referred to the newly established order of the Choano-Flagellata, 

 and distinguished in all instances by their possession of the delicate, collar-like organ 

 which encloses with its base the single terminal flagellum, must undoubtedly be 

 associated with the name of the late Professor H. James-Clark, of the Agricultural 

 College of Pennsylvania, U.S.A., who in June 1866 communicated to the Boston 

 Society of Natural History a detailed account, with copious illustrations, of four 

 American forms discovered by himself as inhabitants partly of salt and partly of 

 fresh water. Previous to this date such authorities as Ehrenberg and Stein had 

 certainly figured and described one or two species of the genus Codosiga as minute 

 varieties or early growths of representatives of the Peritrichous genus Epistylis, 

 while Fresenius had gone so far as to recognize in the form now known as Codosiga 

 botiytis the flagelliferous character of the component zooids, and also the possession 

 by them of a truncate hyaline projection of the anterior border that may be readily 

 identified at the present day with the distinct collar-like organ that characterizes all 

 members of this group. Prior even to the time of Ehrenberg a record of the 

 existence of these minute Flagellate organisms is to be met with, some members of the 

 genera, Monosiga or Sa/pi7iga'ca being in all probability represented by the so-called 

 " squamulse adh^erentes " figured by O. F. Miiller, and even referred to by Leeuwen- 

 hoek as being found attached to the pedicles of Vorticella {Carchesmm) polypinuvi 

 and V. {Episfylis) anastafica. The names of those who have to be accredited 

 with the confirmation and further extension of the comparatively recent discoveries 

 of Professor Clark are as yet but iQ\N in number. The pleasant experience of being 

 the first upon this side of the Atlantic to recognize types belonging to the same 

 category, partly identical with and partly differing from those made known by that 

 authority, and to generally substantiate that interpretation of their structure which 

 he had first submitted, fell to the lot of the present author. 



A notice of such discovery, with illustrations and brief diagnoses of the several 

 forms observed, was communicated to the meeting of the Royal Microscopical Society 

 held on November ist, 187 1, and was published in the following, December, number of 

 the ' Monthly Microscopical Journal.' From that date forward the author's attention 

 has been more especially concentrated upon this highly interesting organic group, 

 the result of such investigation being the registration of over three-quarters out of 

 the total number of fifty or more species now known to science, and described in 



