26 NATURAL HISTORY OF ARTHROPODS. 



A connection still more important exists between these Crustacea and the herring 

 and mackerel fishery of the coasts of Scotland and Norway. The appearance of these 

 fish on the coasts has been found to accompany the presence of innumerable multitudes 

 of the Calanus and other genera of these Copepoda. No other order of Crustacea can 

 therefore compare in economic importance with this. The smallest pools and ditches 

 swarm with them. "Wherever life may find a lodging in the water there they are 

 found, ready to become the prey of all higher animals ; ready, too, by their surprising 

 rapidity of reproduction to keep their number full. Without them the life of the 

 fresh-water fishes would be almost impossible ; and, lacking their innumerable swarms, 

 the schools of herrings and other sea^-fish would hardly be able to exist. 



A few forms of this group demand special notice in addition to those already 

 named. 



One of the marine genera, Sdpphirina, belonging to the Coetc^id^, is among the 

 most beautiful of animals. It is large, — nearly a quarter of an inch 

 long, — and is broad and flat in shape. Below the transparent cuticle 

 it possesses a layer of cells from which it gives off flashes of light. 

 This power it shares with numerous other animals, surpassing them 

 in the brilliancy of the light, and in the variety of the colors. While 

 most such animals shine by night, Sapphirina shines by day. As 

 one looks into the calm sea he may catch sight of brilliant flashes 

 of color in the water beneath him, — purple, sapphire blue, gold, or 

 green, or other hues. The brilliancy may also vary from the softest 

 to the intensest and most vivid tint. Imagine a diamond shining 

 with its own light, and flashing all colors at will, and one may get 

 a faint conception of this jewel of the sea. Like so much of the 

 beauty of the living world, this power of light-emission is connected 

 FiG.32.— saspAirirea. with the mutual attraction of the sexes, and is possessed by the 

 males alone. 

 The families of Copepoda are some twelve in number, and embrace a total of about 

 four hundred species, as at present known. But the field is far from exhausted. 

 Almost nothing is Imown of any except the European species. The American foims 

 have scarcely been touched, while of the rest of the world the Copepoda are an 

 unknown quantity. No field to-day will afford the investigator more novelties than 

 this group of Copepoda. 



Oedbe II. — SIPHONOSTOMATA. 



By far the greater majority of parasitic animals belong to the Arthropods. Land 

 animals are infected with parasitic insects, and the aquatic vertebrates are abundantly 

 supplied with guests and parasites from the class of Crustacea. 



The parasitic Crustacea, like the insects, mainly belong to one order. A few 

 parasites are known among the barnacles, and a few scattered fonns fi-om other 

 groups ; but the Siphonestomata are all parasites, in the broad sense of the word, and 

 the group contains very numerous forms of these unbidden guests. 



The grades of parasitism in the order are very various. Some species are commensals, 

 Uving in digestive tract or gill-sac, but feeding only on the food of the host, not on his 

 tissues. Others, though true parasites, are locomotive, attaching themselves to their 

 prey and sucking his blood, but not permanently residing upon him. Others still are 



