762 ORDER HYPOTRICHA. 



As already indicated in the diagnosis of the family characteristics, the peristome 

 field or oral region is of a more or less triangular or arcuate form, and may be 

 associated with no less than three or even four distinct ciliary series. The most con- 

 spicuous and essential of these series, being indeed never absent, is that bordering the 

 outer or left-hand margin, distinguished by the title of the " adoral " fringe. The 

 constituent cilia of this series are long and powerful, of a cirrate aspect, and admirably 

 adapted for the production, while the animalcule is stationary, of a constant food-laden 

 stream towards the oral aperture, while in the natatory condition they constitute the 

 chief organs of propulsion. According to Sterki,* each individual cilium of this series 

 is much compressed or lamellate, broad at its base, and gradually tapering towards 

 its distal extremity. A similar contour prevails also in the adoral cilia of Stentor and 

 other Heterotricha, and likewise in those of Halteria. As mentioned on a previous 

 page,f it has been proposed by the authority last mentioned to distinguish such 

 lamellate cilia by the name of " membranellse." The second series of cilia, mostly 

 but not invariably associated with the peristome-field, is that fringing its reflected 

 right-hand or inner border, and which is not unfrequently, as in Stylonychia mytilus 

 and Onychodromus grandis, supplemented with or even entirely replaced by a 

 band-like undulating membrane. This second fringe of cilia has received from 

 Sterki the title of the " pre-oral " series. The third series, of still less common 

 occurrence, while indicated by Stein in Pleurotricha and certain species of Oxytricha 

 and Urostyla, but described by him as an elongate groove, conducting to the oral 

 aperture, also owes its correct interpretation to the investigations of Sterki. It 

 occupies a position midway between that of the adoral and pre-oral fringes, and 

 consists of a linear series of exceedingly attenuate vibratile cilia, which taken 

 separately present a close resemblance to, flagella. This central series, which 

 descends posteriorly through the oral aperture into the tubular oesophagus, is 

 denominated by Sterki the " endoral " system. A fourth, but as yet rarely ob- 

 served series has been reported by the same authority as obtaining in Gastrostyla, 

 and this he proposes to distinguish as the " paroral " one. It occurs as an even 

 fringe of fine cirrose cilia, corresponding with those of the pre-oral system, but of 

 still smaller size, produced down the whole inner margin of the band-like border, 

 from the right side of which the powerful adoral cirri or cilia project. The 

 relationship of all the several ciliary systems here enumerated is clearly indicated in 

 Fig. 2 of the accompanying woodcut illustrating the peristomal region of Gastrostyla 

 Steinii, as interpreted by Sterki. A diagrammatic outline of the same region as 

 developed in Stylonychia mytilus, will be found at PI. XLV. Fig. i. 



GENUS I. PSILOTRICHA, Stein. 



Animalcules encuirassed, flattened, more or less oblong; having two 

 rows of long median ventral setae, and a border of similar setae developed 

 around the periphery, but possessing no frontal or anal styles ; endoplast 

 double. Inhabiting fresh water. 



The indurated carapace and absence of the frontal styles serve to distinguish 

 this genus from Uroleptus. 



Psilotricha acuminata, Stein. PL. XLIII. FIGS. 1-3. 



Body shortly oblong, about twice as long as broad, widest and broadly 

 truncate anteriorly, tapering abruptly backwards from the centre, curved 

 towards the left and terminating in an oblique sharply acuminate point ; 

 peristome-field extending nearly to the centre of the body, broadly tri- 



* "Beitrage zur Morphologic der Oxytrichinen," 'Zeit. Wiss. Zool.,' Bd. xxxi., 1878. 

 t Vol. I. p. 65. 



