582 ORDER HETEROTRICHA. 



associated with a minute centrally attached endoplastule ; contractile vesicle 

 of large size, occupying a median position adjacent to the posterior extre- 

 mity. Length 1-300" to 1-96". 



HAB. Mostly fresh water, among Lemna trisulca and other aquatic 

 plants, rarely in sea water. 



Although receiving both its characteristic generic and specific title at the hands 

 of Claparede and Lachmann, this singular species had been already observed by 

 several of the earlier writers, being first figured indeed by O. F. Miiller*as a species 

 of Trichoda, and also as an unknown form by Max Perty. Stein, who in his 

 fine monograph of the Heterotricha gives the most exhaustive description and 

 illustrations of this type, represents it as assuming at will three very distinct and 

 highly characteristic aspects. The first and most normal of these (PI. XXIX. Fig. 6) 

 accords with that given in the foregoing diagnosis, and in which the animalcule is so 

 extended that the entire length is equal to or somewhat exceeds three times the 

 greatest breadth, while the anterior region is twisted obliquely across the body on 

 the ventral side. In the second instance (PI. XXIX. Fig. 7) the body is much fore- 

 shortened, the length now scarcely exceeding twice the breadth, while the peristome, 

 with its row of larger adoral cilia, is caused by this contraction to describe a more or 

 less complete spire around the animalcule's body, and which in some instances is 

 composed of two or even three turns. The third characteristic aspect presented by 

 this animalcule (PI. XXIX. Fig. 8) is produced through its assumption of a com- 

 pletely extended and flattened shape, and in which no trace of the normal oblique 

 flexure of the anterior region being exhibited, its contour corresponds closely with 

 that of a Plagiotoma or Balantidium. It is with this form, which would appear to 

 represent a very distinct local variety if not a separate species, that the largest 

 dimensions recorded in the foregoing diagnosis are encountered. Under the name 

 of Strombidium polymorphum, a great variety of the protean contours assumed by this 

 animalcule have been figured by Dr. Ernst Eberhard in the ' Osterprogramm der 

 Coburger Realschule,' 1862. Among these, however, as already pointed out by Stein, 

 are several representations that must be undoubtedly separated from the present 

 type and referred to the Peritrichous form Gyrocorys oxyura. This type while 

 exhibiting an axial convoluted spire of adoral cilia that closely corresponds with that 

 of Metopus in its second described characteristic stage differs in that the finer cilia 

 of the general cuticular surface, which is of itself indurated instead of soft and flexible, 

 are entirely wanting, the posterior extremity being further supplemented by a long, 

 pointed, tail-like style. The frequent occurrence of these two forms in close associa- 

 tion with each other would seem to favour Eberhard's conjecture that Metopus may 

 possibly represent the earlier or larval condition of Gyrocorys, though such an antici- 

 pation is not supported by the result of Stein's researches. The animalcule described 

 by De Fromentel under the title of Metopus inflatus would appear from the very insuf- 

 ficient details given to correspond with M. sigmoides in that third named and rarer 

 condition in which the oblique flexure of the anterior region of the body is straightened 

 out and obliterated. The occasional occurrence of this species in salt water is 

 recorded by Stein. Possibly, however, such marine example belonged to the type 

 next described. 



GENUS V. METOPIDES, Quennerstedt. 



' Animalcules free-swimming, persistent in form, more or less ovate, the 

 anterior part twisted or folded obliquely towards the left across the 

 remaining portion, the groove between the two regions strongly ciliate, and 

 perforated at its posterior end by the oral aperture ; cilia of the general 

 cuticular surface fine, two or more conspicuous setae developed at the 

 posterior extremity. 



* ' Animalc,,' tab. xxvii, figs, 7 and 8, 1786. 



