GENUS NYCTOTHERUS. 579 



body, its left-hand border bearing an even row of powerful adoral cilia ; 

 cuticular surface with well-marked longitudinal striations, and frequently with 

 finer and less conspicuous interconnecting transverse striae ; cuticular cilia 

 very long, fine, and widely scattered, presenting the aspect of fine setae ; con- 

 tractile vesicles one or two in number, situated near the posterior extremity; 

 endoplast ovate or subspheroidal, centrally located. Length 1-600". 



Hab. — Salt water: White Sea (Mereschkowsky), within the alimentary 

 and radial canals of various small Medusae {^Eucope and Boitgainvillca) and 

 also in certain marine worms {Bradd). 



This species, which is referred with some doubt to the present genus by its 

 discoverer, is remarkable for the great length and setose character of the cuticular 

 cilia ; these appendages, though much less thickly distributed, would appear to 

 correspond closely with the similarly long, fine, setose cilia of the Holotrichous 

 genera Cydidium and Pleurojiema. Probably it will be found desirable to institute a 

 new generic title for its reception. 



Genus III. NYCTOTHERUS, Leidy. 



Animalcules free-swimming, ovate or bean-shaped, more or less com- 

 pressed, the dorsal border convex, the ventral one usually bent inwards 

 centrally ; peristome-field excavate, commencing a little behind the apical 

 extremity, continued in a cleft-like manner on the ventral side to the centre 

 of the body, and there produced backwards and inwards to meet the well- 

 developed ciliated pharynx ; the left border of the peristome only bearing 

 the adoral cilia; anal aperture permanently conspicuous as a chink-like 

 fissure, and continuous with a short tubular rectal passage ; contractile 

 vesicle usually single, subterminal ; endoplast more or less ovate, situated 

 mtsially in advance of the pharyngeal cleft. Occurring as parasites within 

 the intestinal cavities of Amphibia and Invertebrata. 



This genus was founded by Professor Leidy* for the reception of the form 

 hereafter described under the title of Nyclotheriis velox, to which various 

 supplementary species have since been added. From Plagiotoma, which it to a 

 considerable extent resembles, it is to be distinguished by the excavate character 

 of the peristome-field, and by the highly developed, permanently conspicuous anal 

 passage. In connection with this last-named structural difierentiation Ayctot/wrus, 

 indeed, stands almost alone among the entire infusorial series. As will be at 

 once recognized it needs the intercalation of but a short continuation inwards of the 

 inceptive and excretory passages to produce as physiologically complete an alimen- 

 tary tract as is possessed by the Proctuchous Turbellaria and other lower Metazoa. 

 It is a somewhat remarkable circumstance that an Infusorium provided with so com- 

 plex an alimentary organization should, as in the case of Nyctotherus cordiformis, 

 subsist side by side with the Holotrichous Opalinidae, in which all trace even of an 

 inceptive aperture has either remained undeveloped, or become completely 

 obliterated. It might be suggested, in this connection, that while these Holotricha 

 have adapted themselves, like babes in evolution, to an entirely fluid nutriment, 

 Nyctotherus has retained and further developed a partiality for the more solid meats 

 of the rich feasting grounds among which it delights to rove. 



It is desirable to remark that that aspect in Nyctotherus and other allied genera 



' Proc. Acad. Nat. Sei., Philadelphia,' 1849. 



