INFUSORIA. 43 



Sub-Order III. — Peritricha. 



This grouj) resembles the jireceding, ami imlced tlieiv are a few forms whose exact 

 position is doubtful. The typical Peritricha, however, have the cilia confined to a circle 

 around the mouth, while it is only in the aberrant forms that sujiplementary cilia are 

 found. The members of the sub-order are divided into two not very natural divisions, 

 according as they are free or attached, at least during a portion of the existence of the 

 individual. The attached forms frequently develop elegant cases, or in other instances 

 beautiful branching colonies. 



As an example of the first (free) grou]i we may consider the unusually active Ilal- 

 terice, which are globose forms seen in water from jJonds, especially after it has been 

 standing for some days. They have a spiral adoral wreath of cilia for swimming, and 

 usually, in addition, a girdle of long-springing setae, by means of which they leap com- 

 jiaratively long distances. One may be quite under the observer's eye, when to his an- 

 noyance instantly it darts out of the field of view. To facilitate their study Chqiarede 

 and Laehmann recommended placing under the cover 

 with them a form like Acineta. They soon jumj) 

 against its sucking tentacles, where they stick fast, 

 and then may be conveniently examined. Fig. 43 

 illustrates Halteria volvox. It resembles the more 

 common H. grandinella in form and in leaping oi-- 

 gans, but has besides an equatorial zone of long, i-e- 

 curved cilia. A singularly aberi-ant form ap])eai-s in 

 Torquatella tunica, described bv E. Ray Lankester. fio. 43. — //n/to'mTOiTCa;, greatly enlarged. 



-* ^^ . ^ "^ n. Nucleus, cy. Contractile vacuole. 



Around the front margin there is a membranous ex- 

 pansible frill, which is plaited, and alternately closes up and expands with a twisting 

 motion. It was obtained in salt-water from decaj-ing eggs of the worm Terebella. 



Any one having examined the common Hydra, or the gills of .Necturtis lateralis to 

 any extent with the microscope has doubtless encountered minute bodies gliding over 

 the surface of the hosts, or now and then swimming ra]iidly away, but soon returning. 

 Seen from abo\'e they are discs ; from the side, shaped like a dice-box. Prof. H. J. 

 Clark studied carefully their anatomy. He showed that the body surface between the 

 concave extremities is ribbed by the thickenings of the body walls ; that the posterior 

 truncated margin produces a thin annular membrane called the "velum," into the base 

 of which, and on its inner side, the posterior fringe is inserted ; that the nucleus, ex- 

 amined in the fall, was a moniliform, band-like spiral situated near the truncated base. 

 Besides the vibratile cilia of the border of the posterior disc, there is on its inner border 

 a ^\Teath of stout hairs or uncini, in an outer and an inner series ; those of the outer 

 circle are stout and curved, the others, slender, straight, and apparently radiating from 

 the centre of the discal area ; they consist of a solid portion and a membrane-like 

 expansion. These forms belong to the genus Trichodina. 



The family Vorticellid.e includes the attached forms of Peritricha. The three 

 sub-families are Vorticellinae (naked), Vaginicolinffi (loricate), and Ophrydiiiie (in gela- 

 tinous covering). The student of ])rotozoic life must ever find the keenest delight in the 

 study of this varied family, exam])les of which may be found at any time of the year, or 

 at any ])lace where there are natural or artificial bodies of water. In Vbrticella, the 

 type of the family, each individual is solitary, and consists of an oval body attached by a 



