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POPULAR SCIENCE REVIEW. 
no rule as to the method of development ; nearly allied forms 
may present the most diverse conditions, the one passing through 
a larval stage, and the other developing directly in the most 
capricious manner (figs. 9, 13). 
And now that we have reviewed the group of Turbellarians 
generally, we have to speak of one of its two divisions. Leaving 
for a future period the long, almost snake-like Nemertians, we 
pass to the small flattened Planarians, and have to attempt to 
make some of their commonest fresh-water forms known. The 
Planarians or Aproctous (without an anus) Turbellarians are 
found on the sea-shore, in fresh waters, and in the earth. The 
marine species of our own coasts are not abundant, but occur on 
most rocky shores. Some of the fresh-water species occur in 
almost every pond and ditch, whilst one earth-living species is 
known in Europe, and may be expected to occur in Britain. It 
is quite impossible to give a list or descriptions of the British 
species, as no one knows how many there are, so little attention 
has been paid to them. I think, however, a diagnosis of the 
genera which may be expected to occur in ponds or streams, 
will be useful, and a brief description of one or two fresh-water 
species, which, though much smaller and far less brilliantly 
coloured # than the marine forms, are more readily accessible. 
After examining these, the reader will be able readily to trace 
the anatomy of others for himself. On account of the homo- 
logies of the proboscis, mouth, and prseganglionic region in 
Planarians, at present so greatly needing further investigation, 
the genera as they now stand are in many cases based upon 
error. Even in Victor Cams’ Handbuch der Zoologie (1863), 
I find the confusion of mouth and proboscis maintained in the 
generic names, though in the chapter on anatomy it is re- 
nounced. The value of the u proboscis ” as a classificatory cha- 
racter is rendered doubtful by the fact that when absent or 
rudimentary in the adult, it may* be present in the young state. 
Duges and Oscar Schmidt have both made generic names in- 
tended to indicate the position of the mouth, but the meaning 
of these names must now be disregarded. F or the determina- 
tion of species , the reader must do the best he can with the 
papers of Duges and Schmidt ; they are by no means definite, 
and anyone who will devote a few years to the study of our 
British species will do valuable work. 
* Some fresh-water forms were stated "by Max Schultze to contain chloro- 
phyl as a colouring matter, as also Hydra and Stentor. This I have lately 
verified by spectrum-analysis. 
