P^CILOPO0A. 
265 
rostrum*, and at otliers concealed or but slightly visible, fulfils the 
functions of a mouth. There are never more than fourteen feet. 
The shell is very thin and composed of a single piece. They are all 
parasitical. 
We will divide this family into two tribes. 
The first — Caligides, Lat. — is characterized by the presence of a 
shell resembling an oval or semi -lunar shield ; by the number of 
visible feet, which is always twelve, — or fourteen, if we include those 
which Leach considers as such, and which I call inferior antennae ; 
by the form and size of the tenth pairs which are sometimes multifid, 
pinnate, or terminated in a fin, and well adapted at all times, and in 
the adult, for the purposes of natation, and sometimes foliaceous, or 
broad and membranous. The sides of the thorax are never furnished 
with wing-like expansions directed backwards and inclosing the 
body posteriorly. 
Here, the body, exhibiting several segments above, is elongated 
and narrowed posteriorly, terminating in a kind of tail with two 
threads, or as many other salient appendages at the end ; this extre- 
mity is not covered by a segment of the superior teguments in the 
form of a large rounded scale, deeply notched in the posterior margin. 
The shell is at least half the length of the body. This subdivision 
will comprise two genera of Muller. 
Argulus, Mull . 
This genus was at first designated under the name of Ozolus, and 
but very imperfectly described. Jurine, Jun., has since studied its 
type with the most scrupulous attention, followed it throughout all 
its changes of age, and produced a perfect and complete monograph 
of it. He has restored to the genus the original name given by 
Muller. 
The Arguli are furnished with an oval shield, posteriorly emargi- 
nated, covering the body, the posterior extremity of the abdomen 
excepted, and beai’ing on a mediate, triangular space distinguished 
by the name of clypeus, two eyes, four very small, almost cylindrical 
antennae placed in front, the superior of which, shorter and triarti- 
culated, have a stout, edentated and recurved hook at their base ; and 
the inferior quadriarticulated, with a small tooth on the first joint. 
The siphon is directed forwards. There arc twelve feet. The two 
first terminate in a transversely annidated disk, striated and edentated 
along the margin, and presenting internally a sort of rosette formed 
* The composition of this rostrum or beak is not well known. It is evident, 
from the figure of the Argulus foUuceus, given by Jurine, Jun., that it contains a 
sucker ; but is this the case with the others, and of how many pieces is it composed 
I connot answer the question. I presume, however, that this siphon consists of the 
labrum, mandibles and the ligula which forms the sheath of the sucker. In the 
preceding Entomostraca, the four anterior feet, whose form is very different from 
that of the following ones, would correspond to the four jaws of the Decapoda. 
