GENUS PLEURONEMA. 543 



form, and in the manner of its extension and retraction, this membranous trap maybe 

 appropriately compared with the extensile hood of a carriage or an outside window- 

 shade, forming, when expanded, a capacious hood-shaped awning, and when not in 

 use being packed away in neat folds close around the animalcule's mouth. Some 

 distant homological relationship not improbably subsists between the hood-like 

 membrane of Pleuronema and its allies, and the delicate funnel-shaped membrane 

 characteristic of the recently discovered and extensive group of collared flagellate 

 Infusoria described in the preceding volume. 



Pleuronema chrysalis, Ehr. sp. PL. XXVII. FIG. 55. 



Body elongate-ovoid, compressed, from two to two and a half times as 

 long as broad, convex above, slightly concave beneath, subequally rounded 

 at the two extremities ; oral fossa situated near the centre of the ventral 

 surface, followed by a tubular pharyngeal passage ; the extensile membrane 

 or velum largely developed, its depth in full extension corresponding with 

 that of the body. Fine hair-like setae equal in length to one-half of the 

 diameter of the body, developed in even longitudinal rows over the whole 

 cuticular surface ; contractile vesicle anteriorly located ; endoplast sphe- 

 roidal, subcentral. Length 1-360" to 1-192". HAB. Fresh water. 



This species is identical with the Paranuzcium chrysalis of Ehrenberg, but is 

 referred to the present genus by Dujardin under the title of Pleuronema crassa. 

 Its habits are eminently social, closely resembling those of the more minute 

 Cyclidium glaucoma, but it is much less active. When first transferred to the stage 

 of the microscope for examination the animalcules rush to and fro across the field, 

 their setose cilia being the only appendages visible. In a little while, however, 

 they become quiescent, and letting down their delicate hyaline traps or vela, angle 

 for their accustomed food, often remaining anchored, as it were, in one spot for an 

 hour or more at a time. When disturbed, they spring briskly from one side to the 

 other with the aid of their fine body setae, after the manner of Cyclidium. 



Pleuronema marina, Duj. 



Body elongate-ovoid, slightly compressed, three times as long as broad, 

 obtusely pointed at each extremity ; the cuticular surface finely striate 

 longitudinally. Length 1-250". HAB. Sea water. 



Claparede and Lachmann have proposed to regard this form as identical with 

 P. chrysalis, but its average larger size and longer proportions, as given by Dujardin, 

 together with its marine habitat, seem sufficient to warrant its separate specific recogni- 

 tion. According to the figure given by its discoverer, the oral membrane is not 

 relatively so large as in P. chrysalis; the vibratile cilia of the oral system, on the 

 other hand, appear to be more extensively developed. 



Pleuronema natans, C. & L. 



Body subspherical, scarcely longer than broad, oral fossa very large and 

 deep ; extensile oral membrane or velum protrusible to a short distance 

 only ; fine short setae distributed throughout the cuticular surface. Length 

 of body 1-300". HAB. Freshwater. 



