498 ORDER HOLOTRICHA. 



Isotricha (P) microstomum, C. & L. sp. Pl. XXVI. Fig. 38. 



Body subcylindrical, equally rounded at both extremities, slightly con- 

 stricted centrally ; about two and a half times as long as broad ; cuticular 

 surface finely striate longitudinally, entirely clothed with long, fine, even 

 cilia ; oral aperture minute, situated at a distance of one-third of the 

 length of the entire body from the anterior extremity, followed by a short, 

 tubular, backwards-directed pharyngeal tract ; contractile vesicle single, 

 spherical, located a little behind the centre of the right-hand border. 

 Length 1-250". Hab. — Salt water : Norwegian coast (C. & L.). 



This species is referred by Claparede and Lachmann to the genus Paramcecium, 

 from all the typical examples of which it, however, diflers in its symmetrical 

 contour and in its simple tubular oral aperture, unassociated with any adoral 

 groove. This combination of characters accords so closely with those of Stein's 

 genus Isotricha that it becomes requisite either to refer it to that group, or, in virtue 

 of its non-endoparasitic habits, to establish a new genus for its reception. While 

 the former alternative commends itself most favourably for adoption, it must at 

 the same time be observed that the induration of the short tubular cesophagus is 

 alone wanted to convert this t)'pe into a normal member of the genus N'assula, and 

 it is just possible that such a structure has been overlooked by its observers. 

 Among the examples of this animalcule examined by Claparfede and Lachmann 

 several were distinguished by their yellow-brown tint. 



Genus V. HOLOPHRYA, Ehrenberg. 



Animalcules free-swimming, ovate or globose, elastic and changeable in 

 form, entirely ciliate ; the mouth situated at the anterior pole, and the anal 

 aperture at the opposite or posterior extremity ; pharynx simple, having no 

 corneous tube or rod-fascicle ; no specially large cilia developed round 

 the oral aperture ; multiplying by transverse fission, and forming spherical 

 encystments. Chiefly inhabiting fresh water. 



The members of this genus somewhat resemble those of Prorodon, the most 

 essential features of distinction being the unarmed character of the pharyngeal 

 passage and elasticity of the cuticle, which permits the animalcules, as in Cyrto- 

 stomum, to assume a great variety of outlme. 



Holophrya ovum, Ehr. Pl. XXVI. Fig. 45. 



Body ovate, more or less cylindrical, about one and a half times as long 

 as broad ; surface of cuticle obliquely striate or corrugate ; cilia short, fine, 

 and closely set ; colour green or transparent ; oral aperture apical, its border 

 slightly projecting ; contractile vesicle single, spherical, situated posteriorly, 

 close to the anal aperture ; endoplast rounded or ovate, subcentral. Length 

 1-2 10". Hab. — Pond water, amongst Coufervce. 



Although an ovate outline represents the typical contour of this species, it 

 frequently assumes an inflited, subspheroidal shape, the characteristic cuticular 

 striae becoming entirely obliterated ; it is often impossible, under these conditions, 

 even to recognize the position of the mouth, the animalcule having the aspect of 

 a complete sphere, which may be either perfectly transparent or more or less 

 coloured with chlorophyll-granules, or opaque through the ingestion of food-particles. 



