FAMILl OXYTRICHIDAE. 761 



Oxytrichidae. Apart from the peristome and its special series, presently referred to, the 

 modified cilia of the ventral surface may in this form be resolved into as many as five 

 distinct systems or groups. The first of these (fr) occupying the anterior and right- 

 hand region of the ventral aspect, or the one closely corresponding with that relegated 

 on the left side to the peristome-field, is distinguished by the title of the " frontal " 

 series. The normal number of the variously modified frontal cilia in Stylonychia and 

 certain other allied genera is seven or eight, out of which the more anterior three or four 

 are of unusually large proportions and claw-like or uncinate in shape, the remaining 

 smaller ones being stylate or setose ; all of these elements are more or less elastic, 

 capable of flexure in various directions, and fulfil the role of anterior ambulatory 

 appendages. The several genera, Psilotricha, Kerona, Stichotricha, and Stichochata, 

 furnish examples in which the frontal styles are entirely undeveloped or suppressed, 

 while in Stylonethes and certain species of Urostyla the more normal number of 

 eight is considerably exceeded. 



The second and, for the purposes of generic discrimination, most important 

 group is that known by the name of the " ventral " series, and which, as its name 

 implies, occupies the most central area of the ventral aspect. In Stylonychia, as 

 indicated at v in the accompanying woodcut, and in some other genera, this ventral 

 group consists of five setose, stylate, or uncinate cilia only, whose disposition is 

 more or less quincuncial. These five primary ventral cilia are, in a few genera, 

 supplemented by one or more rows of usually smaller ventral setae, or, as occurs in 

 considerably the larger number of genera, is entirely replaced by a greater or less 

 number of such rows. In no member of the Oxytrichidae, as at present known, are 

 the ventral styles or setae entirely unrepresented, and the value of the characters 

 afforded by these elements, as an index to the leading generic subdivisions, becomes 

 apparent on reference to the schedule of the families and genera of the Hypotrichous 

 Order given on a preceding page. 



The anal ciliary series (an), forming the third group, is stationed towards the pos- 

 terior extremity of the body and in close contiguity to the anal aperture. These cilia 

 are mostly of a stylate form, and often of more considerable size than those of the co- 

 associated groups. In the majority of instances, as exemplified in Stylonychia, they 

 are five in number and of a more or less rigid consistence, though not unfrequently, 

 as in Pleurotricha, they are flexible and participate with the ventral and frontal series 

 in the office of perambulation. In four genera only, among the Oxytrichidae, viz. 

 Stichotricha, Strongylidium, Uroleptus, and Stylonethes, is the anal series entirely 

 unrepresented. The fourth or " marginal" ciliary series (m) is composed usually of 

 short setose cilia, which form a more or less even fringe down each lateral border ; 

 these mostly meet together or are uninterrupted posteriorly, and present but 

 a slight amount of variation. In no member of the family is the marginal series 

 entirely absent, though in certain types, such as Amphisia, its normal character of a 

 bilateral peripheral fringe is departed from, the two lateral rows being removed far 

 inwards, close to the two central rows of ventral setae, with which in both size and 

 aspect they closely correspond. The fifth and but rarely represented group is the 

 caudal series (c), consisting of from two to four long hair-like setose cilia, which 

 project in a tail-like manner from the posterior border. In those animalcules in 

 which caudal setae occur, such as Stylonychia, it is worthy of note that the marginal 

 series is posteriorly interrupted, and that the former may therefore be interpreted as 

 representing abnormally developed elements only of the marginal set and not as a 

 wholly independent group. In addition to the from four to five ventral groups of 

 cilia just enumerated, it has been recently shown by Engelmann* that, in Stylonychia 

 mytilus and many other species, a supplemental peripheral fringe of fine immotile 

 hispid cilia is developed externally to the marginal series, these in certain cases 

 e. g. Plagiotricha (Gonostomum) affinis and Oxytricha pelionella being produced 

 likewise in even longitudinal rows over the entire dorsal aspect. 



The peristomal or oral ciliary system of the Oxytrichidae is somewhat complex 



'Zeitschrift fur Wissenschaftliche Zoologic,' Bd. xi., 1861. 



