446 M.T. Thorell on the Species of the Argulide. 
is its proper habitat, since three-fourths of the species which be- 
long to the genus Argulus, together with all the species of the 
genus Gyropeltis, are limited thereto. The greater number be- 
long to the warm parts of that continent: only one species (A. 
pugettensis) is known from the west coast of (North) America. 
Of the four non-American species enumerated in our list, Hurope 
has afforded three, and Africa one species (inhabiting the Medi- 
terranean). 
It is further of importance to note the relative numbers of the 
species which live in fresh and in salt water. We perceive 
at once that, as in the other Branchiopoda, the number of 
freshwater forms preponderates. If we except the four species 
(A. pugettensis, funduli, and elongatus, witheGyropeltis Kollari) 
concerning which information in this respect is wanting, it will 
be seen that of the remaining species only two (A. purpureus 
and giganteus) are found on fishes which live exclusively in the 
sea; all the others are procured from fresh water. It would be 
of great importance to learn whether or not some of these fresh- 
water forms can also live in salt water, and, in particular, whether 
the species (A. coregoni and alose, for instance) which live on 
fishes which undertake journeys from the sea up the rivers 
follow their hosts when these betake themselves again to the 
sea. We have already stated that A. catostomi lives also in the 
brackish water near the mouths of rivers. 
In close connexion with the consideration that the larger 
number of the Argulide belong to lakes and rivers, and only a 
small portion to the sea, is the result which we obtain from an 
inquiry how they are distributed amongst the various groups of 
fishes. That the Argulids are not always (perhaps never) 
confined to a single sort of fish is shown in the case of the 
three European species, which live on fishes of different genera, 
even of widely separated families—especially A. foliaceus, the only 
Argulid of whose habitat and mode of life we have a tolerably 
satisfactory knowledge. This species not only attaches itself 
to freshwater fish of wholly different orders (Acanthopteri, 
Pharyngognathi, and Physostomi), but even attacks the larve 
of frogs, which is not known to be the case with any of the 
other species. We are ignorant as to the animals on which 
three of these (A. pugettensis, A. elongatus, and G. Kollart) 
occur. The rest have all been found on fishes, and, with 
the exception of a single species, on Teleostei or bony fishes. 
Of the other subclasses only one (viz. the Selachia) has figured 
in our list. A. giganteus is found upon a Ray. Amongst bony 
fishes it is, as we might almost conclude @ priori, especially the 
order Physostomi which is affected by these parasites. A spe- 
cies of the Siluroid family harbours G, doradis ; many Cyprinoids 
