356 THE FKESH-WATER LOCHS OF SCOTLAND 
we find a number of Turbellaria (such as Plcmaria)^ which are common 
inhabitants of ponds and streams, but many other genera are terres- 
trial or marine. 
The small group of the Rotifera is an overwhelmingly fresh-water 
one, there being, however, some species found in brackish water, and 
a few which are marine. 
In the case of the Echinodermata, we have a striking example of a 
very large assemblage of forms, not one of which, so far as we know, 
exists outside the sea. It is, in fact, the only instance of a really large 
group confined to one medium without a single exception, and it has 
a special interest accordingly. 
With very few exceptions, the phylum Coelenterata is similarly 
marine. In the subdivision Hydrozoa alone, a very few forms 
inhabit fresh water, the most important being Hydra and Cordylophora. 
Thus, as in the majority of cases we have considered, there are certain 
exceptional forms which are sufficient to disprove any general state- 
ment as to habitat. 
The sponges are almost as strikingly salt-water forms as are the 
Coelenterata. Out of some fifty known families, a single sub-family 
only has fresh-water representatives, but the principal fresh-water 
genus — SpongUla — is widely distributed in the rivers and lakes of 
most parts of the world. 
Amongst the simplest forms of life, the Protozoa, we find a number 
of organisms which are familiar objects in fresh water, and yet the 
majority live in the sea, where they play a very important role. 
Most of those in the sub-groups Lobosa (including Aynoeha and 
Difflicgia) and Ileliozoa {ActinospJioeriiwi^ etc.) have a fresh-water 
habitat ; but the Foraminifera are overwhelmingly marine, and the 
Radiolaria entirely so. Finally, the Ciliata, with forms such as 
Vorticella and Paramcecium, is a sub-group well represented in fresh 
water, and so is the Flagellata, with Euglena ; but both of these 
contain also a considerable number of marine types. 
It will now be evident that we know of some aquatic forms which 
are usually absent from the ocean, in addition to others which are 
seldom or never found in fresh water, and it may be well to enumerate 
again the most striking examples. In the sea we do not find 
Amphibia, Dipnoi, or phylactolagmatous Polyzoa. Further, there are 
in the ocean comparatively few insects and insect larvae, Hydrachnidae, 
Branchiopoda, Oligochaeta, leeches, and Rotifers. On the other hand, 
the following groups do not live in fresh water : Cephalochordata, 
Tunicata, Cirripedia, Cumacea, Stomatopoda, Brachiopoda, Cephalo- 
poda, Polychaeta, and Echinodermata. Besides these, the Elasmo- 
branchs. Decapod Crustacea, Nemertinea, Coelenterata, and sponges 
are only poorly represented apart from the sea. 
