4l6 ORDER FLAGELLATA-EUSTOMATA. 



mass, or whether this central space is simply fluid ; Biitschli inclines to the last- 

 named alternative, having, he says, observed diatoms and other foreign bodies moving 

 freely in this position. The present author is, however, disposed to maintain that 

 this matrix persists throughout, being most fluid centrally and becoming gradually 

 denser as the peripheral region is approached. In both living and preserved 

 examples no trace whatever could be detected of distinctly differentiated internal 

 and external zones. 



From Volvox, Syncrypta, Synnra, and various other free-floating animal and 

 vegetable Flagellate types, for which the colony-stocks of Uroglena volvox are some- 

 what liable to be mistaken, an easily recognized superficial feature of distinction is 

 aftbrded by the general contour of the colony-masses, which rarely exhibit that 

 perfect spherical symmetry which characterizes the several first-named forms. 



Fam. VI. ZYGOSELMID^, S. K. 



Animalcules solitary, free-swimming or repent ; flagella two in number, 

 similar in character, both vibratile ; endoplasm sometimes coloured green, 

 but not enclosing differentiated lateral pigment-bands ; oral aperture 

 distinct, terminal ; one or more eye-like pigment-specks frequently present 

 Mostly inhabiting fresh water. 



Genus I. EUTREPTIA, Perty. 



Animalcules free-swimming or repent, plastic and changeable in form, 

 ovate or elongate ; oral aperture terminal, funicular ; flagella two in 

 number, vibratile, of equal size, issuing from the terminal oral fossa ; 

 endoplasm coloured a brilliant green, enclosing an anterior eye-like 

 pigment-spot ; contractile vesicle anteriorly situated ; endoplast subcentral. 

 Inhabiting fresh water. 



Excepting for the possession of two equal-sized flagella and the exhibition of the 

 pecuhar peristaltic movements presently described, the as yet single known repre- 

 sentative of this genus corresponds essentially with Eiiglcna. 



Eutreptia viridis, Pty. Pl. XXI. Figs. 54-59. 



Body soft and plastic, exceedingly variable in form, its more normal 

 contour when swimming pyriform, broadest and widest anteriorly, with 

 an attenuate and pointed caudal prolongation, at other times elongate 

 and subcylindrical, subfusiform, napiform, or variously constricted, often 

 repent, creeping over the surface of submerged objects by active peristaltic 

 contractions ; flagella slender, equalling the body in length ; eye-like 

 pigment-spot conspicuous, scarlet ; contractile vesicle located close to the 

 last-named structure ; endoplast spherical, subcentral. Length 1-240". 



Hab. — Pond water. 



Since the discovery and description of this animalcule by Max Perty in 1852, it 

 does not appear to have fallen beneath the observation of any other investigator and 

 has been more usually regarded as an imperfectly recorded phase of some one of the 

 various species of Euglciia. As such it is dismissed by Stein in the third volume 

 of his ' Infusionsthiere,' and until quite recendy this verdict was accepted by the 

 present author. So late as February 1879 this type has, however, been encountered 

 in considerable numbers in pond water from near the village of Samares, Jersey, 



