THE LOBSTER-LOUSE. 
485 
VARIOUS-FOOTED ENTOMOSTRACA ; PCEC1EOPODA. 
TUBE-MOUTHED ENTOMOSTRACA ; SIPHONOSTOMA. 
We now come to another group of Entomostraca which are parasitic upon fish and other 
inhabitants of the waters. They belong to Dr. Baird’s third legion, called the Pcecilopoda, a 
term derived from two Greek words, signifying various-footed. They are so named because 
they are partly formed for walking or seizing prey, and partly for swimming and breathing. 
In the first order, the Siphojstostoma, or tube-mouthed Entomostraca, the mouth is furnished 
with a tube containing sharp, spike-like mandibles. The foot-jaws are well formed. The 
object of the tube and its sharp mandibles is evidently for the purpose of piercing the skin 
and sucking the juices of the beings upon which they cling ; and the strong foot- jaws enable 
them to hold so firmly, that they cannot be shaken off. The first tribe is called Peltocephala, 
or buckler-headed, because the head is shaped something like an ancient buckler ; the head 
is also furnished with plates in front, and small antennse of two joints. The first family of 
these creatures is called Argulidge, and may be known by the circular-shaped head-shield, 
and the manner in which the second pair of foot-jaws are modified into a pair of powerful 
suckers. 
The Fish-argulus may be seen upon many of the ordinary river-fishes, the stickleback 
being its favorite. I have seen it on the roach, and even upon the golden carp. It is not very 
small, being about the diameter of a small sweet pea, and may easily be watched if placed in 
an aquarium in which any fish are swimming. The little creature at once makes for the fish, 
darting along with considerable speed, and fixes itself to the side just under the pectoral fins. 
It does not, however, remain fixed to the fish, but occasionally leaves it, and starts off on little 
voyages of discovery, always, however, returning at short intervals, as if for the purpose of 
assuring itself of a meal. It is wonderfully flat, looking very like the shed seed-vessel of 
some plant, and the resemblance is increased by its pale green color. 
The female is considerably larger than the male, and may at once be known by the black 
spot on each side of the abdomen. 
The Caligus is referred to another family. 
This creature is mostly found upon the codfish and brill, and clings with great firmness. 
Mr. Tuffen West tells me that he has examined the Caligus carefully with the microscope, and 
assured himself that the suckers are present. “They are hemispherical, shallow in front, 
where their margin thins off to a translucent membrane ; and deep behind, where their con- 
cavity is bounded by a strong, transversely striated membrane.” 
A remarkable parasite, adherent to the gills of the lobster, is called JSicothoe astad. 
This creature belongs to a different tribe, which may be known by the small and mostly blunt 
head and the long and well- jointed antennae. The family Ergasilidae have the head rounded, 
the body oval, the abdomen well developed, and the feet small and branched. 
The Lobster-louse is sometimes found in considerable numbers fixed to the gills of the 
lobster, from which the female never moves after she has once taken a firm hold, though the 
male is more erratic in his habits, and swims about as he chooses. During her early youth, 
the female is not much larger than the male ; but, as soon as she attaches herself to her new 
home, a pair of strange projections are seen to grow from the side, and by degrees become so 
large, that they seem to constitute the entire creature. Below these projections the egg-sacs 
are developed. 
A curious parasite that infests the sturgeon is rather more than half an inch in length and 
the twelfth of an inch in breadth. It is termed Dichelertium sturionis. This creature insin- 
uates itself deeply into the skin, making its way to the bony arches upon which the gills are 
supported, but not appearing to touch the membranous gills themselves. Sometimes as many 
as ten or twelve are taken from a single fish. They can grasp very firmly by means of their 
forceps, and are able to turn round whenever they please. This curious creature belongs to 
