THE PARASITIC COPEPODA. 217 



in which they are met with is not strange, notwithstanding their many enemies. The Cyclops feeds 

 both on animal and vegetable matter. 



The Cetochilus, or " whale-food," is one of the small Entomostraca, known to the fisherman of 

 the Firth of Forth by the name of maidre, on which the Herring and many fine species of Salmonida? 

 live almost exclusively. These small Copepods abound in such quantities as to obscure the water ; 

 immense shoals of Cod-fishes are seen swimming lazily about, devouring them in large numbers. 

 Shoals of Herrings are also seen pursuing them with great agility. 



The Parasitic COPEPODA may be divided into two groups. The first comprises the free-swimming 

 genera, in which both the male and female retain their organs of locomotion in the adult state, and 

 can change their habitat whenever needful. This division includes the fresh-water Argulus and the 

 marine Caligus. The second division includes the fixed parasites, in which the females, when adult, 

 lose their locomotory appendages, and become fixed, deriving their nourishment by a true suctorial 

 mouth, armed with jaws for piercing the tissues of the fishes and other animals upon which they are 

 parasitic ; the males, however, remaining free. 



" The Argulus in England is found upon various fresh-water fishes. In the neighbourhood of 

 London it is most commonly to be met with upon the Stickleback, but it has been taken also upon 

 the Carp and the Roach ; and in other places it has been found upon the Trout, the Pike, the Perch, 

 and even upon the tadpole of the Frog." 



Professor Dana described a species taken in the Mill River, near Whitneyville, into which the 

 tide runs, thus showing that Argulus can live in brackish water. 



Loefling states that the part where Argulus foliaceus is chiefly found is within the gills, or 

 immediately outside ; and Dana and Herrick inform us that their Argulus catostomi was always 

 found within the branchial cavities, but when the fish itself was immersed in fresh water, the 

 parasite forsook the gills, and, after swimming about some time, would often attach itself to the 

 anterior part of the body. The number of eggs deposited by one Argulus is very considerable. Dr. Baird 

 says as many as 400 have been laid by A. foliaceus, and 1,500 and upwards by Argulus catostomi. 



Caligus and other allied genera are called fish-lice, and are observed to infest the Cod and the 

 Salmon ; they are marine Crustacea. Dana says the Caligi are most numerous on half-grown fish, 

 and occur on the head and different parts of the body, but never within the gill-covers or under the 

 scales. Dr. Baird says of the European species that they live under the scales, and are often found 

 on the parietes of the mouth and branchial cavities. " When disturbed, they move with rapidity 

 over the fish, and either backward or forward with nearly equal facility. In swimming, their motion 

 is equally rapid. They thus travel over the fish at will, and, we do not doubt, occasionally leave one 

 fish for another." 



Both sexes frequently occur on the same fish, though the females are the more abundant. The 

 sizes of the individuals vary, but the adult male often is two thirds of an inch in length. The 

 females are seldom more than half an inch long, and are always smaller than the males. 



" The Caligi live several hours on the body of the Cod taken from the water, but generally die 

 soon after the death of the fish. When taken from the fish and confined, they exhibit a strong 

 inclination to leave the water. These animals, like the Cod on which they live, require a low 

 temperature, and have been observed to swim, with scarcely diminished activity, in water that was 

 freezing. In some instances, when the water had evidently reached a temperature below 32 Fahr. 

 without congelation, they have been rendered torpid, and apparently dead ; but on bringing them 

 into a room not above 45 Fahr., they have soon resumed their usual activity." 



The Caligi change their skin, as well as the other Entomostraca, but little is yet known of the 

 process. The young, when first hatched, closely resemble the young Cyclops, and, like them, undergo 

 a series of moults, or changes of skin, before they become perfectly developed. 



Nicotlwe astaci, a very small species, of a rosy colour, attaches itself to the gills of the common 

 Lobster (Fig. 39). 



The LERNEAD.E (fixed parasites) fasten themselves to the eyes and various parts of the bodies of 

 fishes in different ways some by means of the foot-jaws alone, others by a series of horns proceeding 

 from the side of the head, and others, again, by two long appendages, which spring from the upper 

 part of the thorax, which unite at the tip, and form a, sort of round button. 

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