GENUS METOPUS. 58 1 



Nyctothems velox, Leidy. Pl. XXIX. Fig. 5. 



Body ovate, white and translucent, the front border obtusely rounded, 

 posteriorly somewhat angular and pointed, enclosing an anterior granular 

 trapezoidal area ; peristome directed abruptly downwards and inwards ; 

 pharyngeal cleft not equalling the peristome in length ; anal aperture con- 

 spicuous, postero-terminal, presenting the form of an inwards and down- 

 wardly produced tubular cleft ; cuticular striae forming a single uninter- 

 rupted longitudinal series ; contractile vesicle large, spherical, postero- 

 terminal ; cndoplast ovate. Length 1-254" to l-lSo". 



Hab. — Intestinal tract of Juliis marginatus. 



Thousands of examples of this animalcule are reported by Professor Leidy as 

 being frequently found infesting the intestinal canal of a single individual of the 

 above-named American myriapod. A form apparently identical with this species, 

 has been more recently obtained by D'Udekem from the Y.\iro]}e3.n /iiliis terrtstris. 

 While the contour of the peristome in N. velox accords most nearly with that of 

 Nyctothems gyoeryanus, the cuticular striation and proportionate size of the pharyngeal 

 cleft resemble that of N. ovalis. 



Genus IV. METOPUS, C. & L. 



Animalcules free-swimming, highly clastic and changeable in shape, 

 normally elongate-oval or fusiform, rounded at both extremities, cylindrical 

 or only slightly flattened ; the anterior portion usually twisted obliquely 

 towards and overlapping the left side of the ventral surface, sharply sepa- 

 rated from the posterior portion ; peristome-field furrow-like, commencing 

 on the left side at a little distance from the anterior extremity, produced 

 obliquely downwards towards the right in the groove formed by the oblique 

 curvature of the body, and terminating in a short pharynx at about, or 

 shortly past, the median line ; on the contraction or shortening of the body, 

 the peristome with the pharynx, for the time, describes a complete spiral 

 circuit, the animalcules presenting in this condition a totally different 

 aspect ; anal aperture postero-terminal ; contractile vesicle single, posteriorly 

 located. Inhabiting salt and fresh water. 



The singular spiral twist of the body exhibited by the animalcule of this genus 

 seems to be to some extent foreshadowed by those species of the genus Nyctothems 

 in which the muscular fibrilla; of the antero-posterior cuticular surface take opposite 

 directions, such peculiarity being most prominently exemplified in the form 

 described under the tide of N. gyoeryanus. 



Metopus sigmoides, Miill. sp. Pl. XXIX. Figs. 6-9. 



Body in its normal aspect cylindro-fusiform, three to three and a half 

 times as long as broad, the two extremities rounded, the anterior one 

 twisted on the ventral aspect obliquely towards the left ; peristome-field 

 produced backwards to the centre of the body, and then continued inwards 

 as a short, tubular pharynx, a tuft of longer cilia developed at the posterior 

 extremity ; endoplast ovate, situated near the termination of the peristome, 



