448 ORDER CILIO-FLAGELLATA. 



of the species. Under these last conditions the cyst produced is mostly found 

 to exhibit a peculiar modification of contour. In place of being subspheroidal 

 or ovate, as obtains under ordinary conditions, one or both of the extremities are 

 prolonged into an attenuate, curved, and acuminate point. These recurved points 

 in the latter instance being produced in the same plane, the result is a crescentic 

 or lunate-shaped cyst, somewhat resembling in shape the Desmid Closterium. 

 Examples of these lunate or subcrescentic cysts are reproduced from Claparede and 

 Lachmann's drawings at PI. XXV. Figs. 47, 48, and 49, 50; those in the two former 

 instances representing a marine, and in the two latter ones a fresh-water type, whose 

 specific identity could not in either case, unfortunately, be precisely determined. The 

 delineation given at Fig. 49 is of special interest, since in that instance the primarily 

 enclosed single zooid has become metamorphosed by linear or serial segmentation into 

 no less than eight sporular elements or daughter-cells, having each the characteristic 

 trilobate contour of the parent, but wanting the investing carapace ; they are in fact at 

 this early phase of their development indistinguishable from the members of the genus 

 Gynmodinium, previously described, and with which they are thus shown to be inti- 

 mately and phylogenetically related. It is further noteworthy, in this connection, 

 that the assumption of a similar and comparatively simple Gymnodinium phase is 

 frequently resorted to by the animalcules of the present genus as a preliminary to 

 the process of encystment, while it is only under such temporarily naked state that 

 increase by fission, or genetic union, can be accomplished. This assumption by the 

 members of the genus Peridinium of an alternating encuirassed and naked state con- 

 siderably enhances the difficulty of determining which forms possess a sound claim 

 for relegation to the genus Gymnodinium, and it is necessarily only those types 

 which are found to persistently maintain the naked condition that can be allotted to 

 this group. 



Peridinium tabulatum, Ehr. sp. PL. XXV. FIGS. 1-5 AND 55-57. 



Body ovate or subcircular as seen in dorsal or ventral view, much 

 flattened or depressed, with a convex dorsal and concave ventral surface, as 

 seen in lateral aspect ; cuirass composed of numerous polygonal facets, 

 which individually exhibit under high magnification a delicate reticulate 

 structure ; colour yellowish green or brown ; one or more red eye-like pig- 

 ment-spots frequently but not invariably developed. Length 1-570" to 

 1-430". HAB. Fresh water ; gregarious. 



This species, originally described by Ehrenberg as a species of Glenodinium, 

 is one of the most cosmopolitan members of the present genus, and frequently 

 occurs in pond-water in such abundance as to impart to the water its own charac- 

 teristic rust-brown hue. A considerable amount of variation is found to obtain in 

 the contour of the two apical poles of the cuirass of this species in examples derived 

 from diverse localities. Sometimes, as shown at PI. XXV. Fig. i., both of these 

 poles are uniformly smooth and rounded ; more often, however, it is found that the 

 facets of the anterior segment of the cuirass project apically in such a manner as to 

 impart a notched or shortly bifurcate aspect to the anterior border, and so prepare 

 the way to the closely parallel but more decided horn-like projections in this region 

 that are found in Ceratium divergens. Examples with a rounded posterior and 

 distinctly notched anterior pole are delineated at Figs. 55-57. A still more 

 complex modification of the cuirass is included in the representations of this species 

 given by Ehrenberg, in some of which, as reproduced at PL XXIV. Fig. 2, the 

 posterior segment is likewise produced apically into three or four acuminate points. 

 The number of large polygonal plates or facets reckoned by Stein to enter into the 

 composition of the two segments of the cuirass in this species, is no less than 

 twenty-one, sometimes more, and of which fourteen to sixteen belong to the anterior 

 and seven to the posterior moiety; added to this, he says, are smaller linear or band- 



