760 



ORDER HYPOTRICHA. 



setae occasionally developed upon the lateral margin, and more rarely on 

 the dorsal aspect ; oral aperture debouching upon the posterior angle of the 

 peristome-field, followed by a short tubular unarmed pharynx ; the anal 

 aperture ventral, located at a little distance from the posterior extremity ; 

 contractile vesicle single, endoplasts usually two or four in number. 

 Inhabiting salt and fresh water ; movements ambulatory or natatory. 



With the Oxytrichidse we arrive not only at the most highly specialized group of 

 the Hypotricha, but, in many respects, at that also of the entire class of the Infusoria- 

 Ciliata. Structural differentiation is manifested more particularly in this family in 

 the remarkable development and relegation to varied purposes of the appendicular 

 or locomotive organs, and which, while in all instances more or less complex modifi- 

 cations of ordinary cilia, exhibit mostly a wide departure from the structures bearing 

 that title as met with in the orders previously described. By many earlier writers, 

 indeed — as explained more at length in the introductory chapter, see pp. 64 and 65 — 

 these appendages were regarded as distinct organs, and have been distinguished by 

 the respective titles of setae, styles, and uncini, &c. ; these terms are, however, in this 

 treatise retained only in an adjective or qualitative sense. Although at first sight 



Fig. I. 



^ar 



Fig. I. — Diagrammatic ow\X\x\e.cti Stylotychia mytilus, illustrating the several ciliary systems ;/, peristomal ciliary 

 series ; fr, frontal, v, ventral, an, anal styles ; tn, marginal, and c c, caudal sets. 



Fig. 2. — Peristomal ciliary system o( Gasirosiyla Steinii {a.hcr Sterki) ; ad, adoral, pro-, prasoral, par, paroral, and 

 eii, endoral series ; yV and ;«, isolate! elements of the frontal and marginal systems. 



there appears to be but little uniformity in the character and distribution of the 

 cilia upon the under surface of the more highly differentiated members of this 

 family, it will be found on closer examination that a certain common plan under- 

 lies and is predominant throughout. What this plan is, is best understood by a 

 brief study of the formula of arrangement exhibited in such a type as Stylonychia 

 mytilus, diagrammatically represented in Fig. i of the annexed woodcut, and which 

 is found to include all the elements, suppressed in some instances and exaggerated 

 in others, that occur in any of the numerous genera belonging to the group of the 



