GENUS PERIDINIUM. 447 



GENUS V. PERIDINIUM, Ehrenberg. 



Animalcules free-swimming, encuirassed ; body divided by a transverse 

 ciliated furrow into two equal or subequal moieties ; a second, shorter, non- 

 ciliated groove produced from the centre of the ventral aspect of the 

 equatorial furrow towards the apical extremity ; cuirass consisting of two 

 primary anterior and posterior segments, which are further subdivided into 

 a variable number of smaller smooth or variously sculptured polygonal 

 facets ; flagellum single, inserted close to the oral aperture near the junction 

 of the equatorial and vertical furrows; a coloured eye-like pigment-spot 

 frequently developed. Inhabiting salt and fresh water. 



The genus Peridinium, as here constituted, embraces most representatives of 

 the genera Peridinium and Glenodinium of Ehrenberg, but from out of both of which, 

 as previously explained, are eliminated and associated with the last-named title those 

 forms only in which the cuirass or carapace is composed of two simple undivided seg- 

 ments. The ciliary groove and wreath of Peridinium and its allies, although usually 

 described as forming an uninterrupted girdle around the centre of the animalcule's 

 body, is shown by Claparede and Lachmann to deviate more or less conspicu- 

 ously from so simple circuitous a course. As made apparent more especially by 

 those observers in connection with the marine form Peridinium spintferum (see 

 PL XXV. Fig. 15), the left extremity or limb of this ciliary girdle takes its origin 

 from a point considerably in advance of the precise centre of the ventral surface, and 

 after obliquely encircling the dorsal region of the animalcule's body, terminates again 

 in the middle line of the ventral aspect, but at a point as much to the rear of the 

 same imaginary centre ; the right and left limbs of the groove, with its accompanying 

 cilia, lie consequently on two distinct levels. A similar deviation of the course of 

 the equatorial groove and ciliary girdle is in a less marked degree being some- 

 times scarcely perceptible shared by all the representatives of the several genera, 

 Peridinium, Glenodinium, and Ceratium, Another characteristic common also to 

 the majority of these closely allied encuirassed forms, is the presence on the 

 ventral surface, and immediately opposite the anteriorly produced non-ciliated 

 vertical furrow, of a deep inlet or emargination in the substance of the posterior 

 segment of the cuirass, which consequently leaves the body of the animalcule 

 naked and exposed. It is within or in the immediate vicinity of this circumscribed 

 area that the oral aperture would appear to be situated, though its existence has 

 not yet been so clearly demonstrated as in the case of Gymnodinium. Imme- 

 diately above, and in advance of this emargination, will be found inserted the single 

 long flagelliform appendage. 



Although the reproductive phenomena of the present genus, and of the Peridiniidse 

 in general, have not as yet been completely elucidated, some important data in this 

 connection have been recorded, through the joint work of Messrs. Claparede and 

 Lachmann. Encystment has been observed by these authorities to take place in a 

 large number of forms and under diverse conditions; segmentation on a more 

 or less extensive scale being in many cases the direct product of the process. In 

 some instances, as shown at PI. XXV. Figs. 4, 10, and 29, encystment is effected 

 within the normal carapace or cuirass, and is not attended by any supplementary 

 metamorphoses. Other instances were observed, however, in which the cuirass of the 

 typical motile zooid being cast off, a new and continuous cyst, comparable in all 

 ways to that produced by the ordinary Flagellate and Ciliate Infusoria, was secreted. 

 The animalcule, pending the secretion of this second envelope, temporarily maintains 

 a naked phase of existence, which is, as shown at PI. XXV. Fig. 5, directly compar- 

 able to the permanent form of Gymnodinium. This variety of encystment is some- 

 times, as among the ordinary Infusoria, associated only with the temporary retreat 

 or hibernation of the animalcule, or it may have as its object the further propagation 



