46 
The Irish Maturalist* 
I^ebruary, 
that Polycelis nigra inhabits the placid waters of ponds, 
where a ghde along the surface can be enjoyed, and floating 
particles of food found. The former lives in rapid streams, 
where there is not even a sporting chance of the surface 
film remaining sufficiently intact for any length of time 
to allow an excursion upon it. Thus, the habits of 
animals are largely moulded on past experience. Pearl, 
a well-known worker on this subject, says you must know 
your planarian as you know a person, intimately, before 
you can safely dogmatize on its behaviour. 
I do not intend dealing with details of anatomy, but 
among other peculiarities of this group of animals the 
position of the mouth can be regarded as a feature of 
popular interest. This aperture lies about half-way along 
the body on the under surface ; it does not lead directly 
to the food channel, but into a cavity in which hangs the 
tubular beginning of the gut like a bell. This tubular 
structure is capable of considerable extension, and can be 
protruded far out of the mouth, trunk wise, ready to secure 
any morsel of food that is about ; after seizing the food, 
this pharynx is retracted through the mouth. A drowned 
earthworm is a much appreciated find for Polycelis cornuta, 
and numbers of these planarians may be seen enjoying 
such a feast. The white protruded pharynx is often 
to be seen in the small black form so common in this dis- 
trict (Polycelis nigra). 
Except as regards the eyes, the sense organs of pla- 
narians are puzzling structures, and may be omitted here. 
The eyes, however, vary in number from none to thousands 
within the Triclad group ; when present in large numbers, 
as in many land planarians, they appear as small dots 
along the front and sides of the head, along the sides of the 
neck, and even for a considerable distance along the sides 
of the body. They can scarcely be regarded as more than 
light perceivers — a real image is not formed by them. 
Of the four common local species mentioned, three have 
two eyes which are rather highly organized, and one 
(Polycelis) has about a score or more arranged round the 
front end and for a short distance along the sides of the body 
in a single row. 
