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; 
— 1g00-1901.] Report of the Microscopical Section. 225 
From the caudal extremity of the Apus are given off two 
_ long appendages composed of a great number of articulations. 
_ Indeed, the outstanding feature of the creature is the immense 
number of its articulations. “Latreille says that we may 
safely take them to be not less than two millions!” It is 
needless to add that we did not attempt to verify this 
statement. 
The Apus has not been found in Britain for many years,— 
not since before Baird’s time,—but it occurs in various places 
in Europe. 
_ Argulus—The Argulus, which next claimed attention, is 
a parasite of various fresh-water fishes, being found upon 
the stickleback, carp, roach, and some others. The name 
of the order, Siphonostoma, in which it is placed by Dr 
Baird, is founded upon the peculiar siphon-like tube with 
which the mouth is furnished to enable the creatures to suck 
the juices of the hosts upon which they live; while the name 
of the tribe, Peltocephala, is founded upon the shield - like 
covering of the head. 
Compared with the Apus, the Argulus is a small crustacean. 
It is of an oval form, measuring about one-fourth of an inch 
in length. The body is covered by a carapace which in the 
tergal aspect stops near the commencement of the thorax, but 
in the lateral aspects is prolonged for a considerable distance. 
This carapace is very transparent, and, like that of the Apus, is 
_ composed of three layers. In the median layer of the lateral 
portions is seen a system of opaque tubes, arising from a single 
branch which springs from the stomach. They seem to fulfil 
the same purpose as the shell glands in the Apus. The two 
_ eyes are situate in the anterior part of the tergal aspect of 
‘the head, and are sunk in the thickness of the shell, They 
appear like two small dark spots, and are areolar. 
_ The apparatus of the mouth is somewhat complex, the most 
prominent part being the siphon or sucking-tube, on which the 
name of the order is founded. This sucking-tube is enclosed 
_ in a sheath which is a prolongation of the lip. The antenne 
are situate one on each side of this tube, but are completely 
covered, when viewed from the tergal aspect, by the carapace. 
But besides the appendages just referred to, there are what 
% Dr Baird describes as the second and third foot-jaws, the 
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