GENUS GYMNODINIUM. 443 



envelope, the animalcules assume after death various irregular shapes, and speedily 

 undergo complete disintegration. As indicated by Stein, Gymnodinium may be 

 regarded as the ground or stock form of the various more complex representatives 

 of the Peridiniidje. 



Gymnodinmm fuscum, Ehr. sp. Pl. XXV. Figs. 17 and 18. 



Body oval, slightly compressed, rounded posteriorly, acuminately pointed 

 anteriorly, nearly twice as long as broad ; surface smooth, colour light brow^n ; 

 no eye-like pigment-spot. Length 1-430" to 1-280". Hab. — Fresh water. 



In the figures of this species, as given by Ehrenberg in connection with the 

 title of Peridiiiiinii' fuscin/i, and here reproduced, the flagellate appendage was 

 apparently overlooked, while the vertical as well as the transverse groove is 

 represented as bordered with cilia. The presumed example of longitudinal fission 

 delineated by the same authority (see PI. XXV. Fig. 18), is regarded by Stein 

 as an advanced phase of the process of fusion or genetic union between two 

 previously independent zooids, which he has frequently observed to take place under 

 similar auspices in connection with the species next described. 



Gymnodinium pulvisculus, Ehr. sp. Pl. XXV. Figs. 19 and 20. 



Body subspheroidal, slightly trilobate, surface smooth ; flagellum con- 

 spicuous, two and a half times as long as the body ; colour light brown or 

 greenish-yellow. Length 1-2300" to 1-1150". 



Hab. — Amongst ConfervcE with Chlamydomonas pulvisculus, often in 

 great numbers. 



As represented in Ehrenberg's figures of this species, coloured matter is freely 

 incepted. Perty reports the presence of an eye-like pigment-spot in the anterior 

 body-half, but such a structure is not indicated in Ehrenberg's original drawings and 

 descriptions. The process of conjugation or fusion has been observed by Stein, who 

 identifies with this phenomenon the representations of presumed longitudinal fission 

 given by Ehrenberg. 



Gymnodinium corpusculum, Perty sp. 



Body ovate or elliptical, segments very unequal, the posterior one con- 

 siderably the smaller ; flagellum conspicuous, about twice the length of the 

 body ; no eye-like pigment-spot ; enclosing variously coloured granules. 

 Length 1-1120". Hab. — Fresh water, among MarcJiantia polymorpha. 



Described by Perty * under the title of Peridiniiim corpusculum. 



Gymnodinium monadicum, Perty sp. 



Body ovate or subspheroidal, segments unequal, the hinder one being 

 considerably the smaller of the two ; eye-like pigment-speck distinct, sub- 

 central ; flagellum conspicuous, two or three times as long as the body ; 

 endoplasm colourless, enclosing green food-corpuscles. Length 1-1120". 



Hab. — Pond water. 



This species was reported by Perty from the Bernese Alps. Stein f has encoun^ 

 tered an apparently identical form in the neighbourhood of Prague, recording of it 



* ' Zur Kenntniss kleinster Lebensformen,' Berne, 1852. 

 t 'Infusionsthiere,' Abth. iii., 1878. 



