258 T. W. Engelmann on the Infusoria. 



Chasmatostoma reniforme (PL III. fig. 16) 



has probably been often confounded with Colpoda cucullulus 

 and similar Ciliata. Anteriorly the reniform body rather tapers, 

 but posteriorly it is obtusely rounded-off. A short oesophagus 

 extends inwards and to the right side from the oval mouth, 

 which is further furnished along its right side within its margin 

 with an actively undulating fine membrane. Sometimes a second 

 smaller membrane of the same kind is imperfectly seen on the 

 left side. The nucleus is circular, placed in the median line in 

 the posterior half of the body, and having a clearly defined 

 round nucleolus occupying a hollow on its surface. The con- 

 tractile vesicle is situated at the posterior extremity, and in its 

 contraction acquires a stellate figure. It swims actively, mostly 

 in a circuitous direction, by means of its uniform cilia, usually 

 with its ventral surface downwards. Fission not observed. 

 Size O06 millimetre. 



Microthorax. PI. III. figs. 17, 18, 19. 



Body loricated, compressed, oval, widely rounded posteriorly, 

 uniformly ciliated. Mouth placed within the concavity of a 

 rounded peristome situated in the left half of the body, and 

 close to the posterior extremity. Nucleus and contractile sac 

 single. 



M.pusillus. PI. III. fig. 17. 



One of the smallest holotrichous Infusoria. It often occurs 

 in large numbers together and in company with small Chilodons, 

 with Cyclidium glaucoma, Cinetochilum margaritaceum, and Pleu- 

 rochiliclium strigilatum, and it is most closely allied to the two 

 last-named animalcules. The left border of the rigid body is 

 straight, whilst the right is convex. The peristome-depression 

 is on the right side, and has an active valvular membrane within 

 it. The right margin of the body is much thickened ; and along 

 the ventral aspect are two furrows, commencing from the ante- 

 rior extremity, on the left side, and traceable to the middle of 

 the body. The dorsal surface is not striated, but bears cilia. 

 The round nucleus is central; the contractile vesicle near to and 

 above the peristome, on its right side. Individuals colourless, 

 their largest dimensions only O03.2 mill. Movements active ; 

 swims on the ventral aspect. 



M. sulcatus. PI. III. figs. 18, 19. 



Much larger than the preceding, but of the same form. Along 

 the dorsum are three deep longitudinal furrows; and the ventral 

 sulci extend to the hinder border. The contractile vesicle is on 

 the left side, and immediately above the peristome, fewiuia 

 actively, on its ventral aspect. Size O057 mill. 



