THE LOBSTER-LOUSE. 485 



VARIOUS-FOOTED ENTOMOSTRACA ; PCECILOPODA. 



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 Poscilopoda, 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 SIPHONOSTOMA, 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 antennae of two joints. The first family of 

 these creatures is called Argulidae, 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 Nicothoe astaci. 

 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 LOBSTEK-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 



