GENUS CYCLIDIUM. 545 



surface ; contractile vesicle postero-terminal ; endoplast spheroidal, sub- 

 central. Length of body i-i 250". Hab. — Pond water and infusions. 



This species is one of the first Ciliate animalcules that makes its appearance in 

 hay or other vegetable infusions, in which it often swarms in countless numbers. 

 In addition to being essentially social in its habits, a certain community of action 

 appears to be maintained between the numerous individuals that are found together. 

 In this manner the springing to one side of a single zooid in the crowd is sure to 

 be responded to by another one in its vicinity, and the whole assemblage is pre- 

 sently set whirling and sporting together like a colony of house-flies in the genial 

 sunshine. As with flies, moreover, a strong light apparently offers a remarkable 

 attraction to these animalcules, it having been observed on many occasions that 

 they crowd to that portion of the microscopic field which is illuminated by the small 

 but brilliant pencil of light produced by the achromatic condenser when employing 

 a -j-V-iiich object-glass, and follow it as different portions of the field are brought into 

 its focus by the stage movements. This tendency to assemble in the area of 

 strongest illumination was exhibited to an equal extent during the employment 

 of either lamp-light or ordinary daylight. 



When actively darting about or swimming leisurely forwards, the characteristic 

 membraneous trap or velum of this species is, as in the case of Pleuronema, stowed 

 snugly away in transverse folds around the margin of the mouth, but is at once 

 lowered again to its full extent when, the moment's sport being over, the little 

 creatures come to anchor. The rapid multiplication of this form is chiefly 

 accomplished by the process of transverse fission, in connection with which it has 

 been observed that the characteristic membraneous velum takes its share. In young 

 individuals developed from germs, it was, however, ascertained by the author that 

 the extensile membrane, as in Uroncma, does not make its appearance until the 

 animalcule has attained its adult dimensions, its aspect prior to this matured condi- 

 tion closely resembling that of Dujardin's Enchelys triquetra, and which, in com- 

 pany with the Alysciun saltans of that authority, and the Pleurotiema cyclidiuvi of 

 Claparede and Lachmann — wrongly represented as having the contractile vesicle 

 and characteristic seta at the anterior extremity — must undoubtedly be identified 

 with the immature state of the present species. Presuming the last-named form 

 to be distinct, Stein has proposed to confer on it the title of Cyclidium ClaJ^aredi. 



Cyclidium citrellus, Cohn sp. Pl. XXVII. Fig. 59. 



Body ovate or lemon-shaped, somewhat compressed, flattened ventrally 

 in the neighbourhood of the subcentral oral aperture, tapering abruptly at 

 the two extremities, about twice as long as broad ; surface of the integu- 

 ment faintly striate longitudinally, cuticular setse fine and evenly distributed, 

 not equalling the breadth of the body in length ; one long and two shorter 

 caudal setae developed at the posterior extremity ; contractile vesicle 

 postero-terminal. Length 1-600". Hab. — Salt water. 



The presence of the two supplementary caudal setae, as well as the symmetrical 

 lemon-like contour of the body of this species, seem to distinguish it readily from the 

 preceding form ; by Cohn, who describes it under the title of Pleuronana citrellus^ 

 the hood-shaped oral membrane is represented as a mere long, recurved seta. 



Cyclidium arboreum, Ehr. 



Body minute, suborbicular, ventral border slightly excavate, dorsal 

 surface rugose; cilia hair-like, scattered throughout. Length 1-1200". 

 Hab. — Among moss on trees. 



