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POPULAK SCIENCE REVIEW. 
in most ponds and streamlets. They are best observed first 
with a lens — in a bottle of the water containing them, and then 
afterwards picked up in a tube drawn out to a fine opening. 
The commonest forms belong to the genera Mesostomum, 
Vortex, and Prostomum (figs. 5, 6, 7, 8). The species are at 
present quite undefined. 
The marine Dendrocoels I have omitted. They are very 
beautiful in colour and graceful in form — many species are 
figured in the memoirs by De Quatrefages and Claparede (fig. 14). 
The commonest little black fresh- water Planaria belongs to the 
genus Poly cells, having many eyes ; species of true Planaria are, 
however, abundant enough with them (figs. 17, 18). All these 
Dendrocoels are much larger than the Rhabdocoel forms, being 
often more than half an inch in length, and very conspicuous by 
their dark colours. Duges, in his memoirs ( 1828-1830), de- 
scribes several species, and also the wonderful power of repara- 
tion of injuries they possess. By slicing them with scissors, 
individuals may be produced with two heads or two tails, and 
otherwise modified. It is Oscar Schmidt, however, who within 
the last seven years has described their anatomy. 
Duges found the very remarkable Geoplana terrestris (fig. 16) 
in France in Languedoc, and it ought to occur in England. 
Will the readers of the Popular Science Review hunt it out ? 
I now propose to speak of the anatomy of Opistomum , a 
Rhabdocoel, and Polycelis , a Dendrocoel, in greater detail. 
Having caught sight of an Opistomum (fig. 5) or Me- 
sostomum (fig. 7) swimming over the surface of a bottle of 
pond-water, you may conveniently catch the minute creature by 
making use of a dipping tube with a fine orifice, and then place 
it on a compressorium or under a glass cover. The first thing 
to note is the ciliation of the whole surface of the body, 
excepting the sucker-like mouth, which in Opistomum will be 
readily seen lying very far back near the posterior extremity 
(fig. 6). A little closer examination of the clear transparent 
skin will disclose the presence of very numerous clear ob- 
long corpuscles, which are scattered all over it, and belong to 
that group of organs which is represented by the “ trichocysts ” * 
of Infusoria and the nettle-cells of Polyps and other animals. 
Examining now the digestive organs, it is not difficult to trace 
the large cylindrical and muscular pharynx (ph) opening from the 
mouth and leading into a capacious but simple sac which has no 
other opening — this is the stomach and intestine both. Coloured 
granular masses are crowded on its surface, and some of these, 
perhaps, act as a sort of bile-forming organ. Overlying this 
stomach are other masses of cells and muscular fibres, of the 
* See Allman, Jour. Micros. Sci. (1855), vol. iii. p. 177. 
