22 NATURAL HISTORY OF ARTHROPODS. 



SUB-CLASS II. ENTOMOSTKACA. 



BETWEEN the Cirripedia, or barnacles, which we have just left, and the Podoph- 

 thalmia, or stalk-eyed crustaceans, there are placed, in all systematic treatises, a large 

 number of Crustacea, mostly of minute size, the large majority of them being so small 

 as to require the compound microscope in their study. Beyond the fact that they 

 occupy the intermediate position just mentioned, authorities do not completely agree 

 in their classification. But this difference of opinion arises chiefly from the fact that 

 some consider the divisions Cladocera, Copepoda, etc., as sub-classes equivalent in rank 

 to the Cirripedia and Podophthalmia, while others regard these groups as only of 

 ordinal value, a view which we are inclined to adopt. 



In the Entomostraca the abdomen is almost always devoid of appendages, and, 

 further, is frequently itself reduced to a mere rudiment. But three pairs of morpho- 

 logical limbs function as mouth-parts, while the true limbs show an almost endlessly 

 varied series of form, and the variations in number are nearly as great, all being lacking 

 in the adults of some parasitic forms, while in some of the Phyllopoda the number 

 of appendages exceeds sixty. Some of the Entomostraca have specialized respiratory 

 organs, in others all traces of organs of circulation and respiration are wanting. The 

 Entomostraca almost always hatch from the egg as nauplii. Beyond these few points 

 it is difficult to go ; for there is so much variation in the different orders and indi- 

 viduals that but few statements will apply to the sub-class as a whole. 



From an economic standpoint the Entomostraca are important only as they 

 indirectly affect human interests by furnishing food for fishes, or, by the parasitic 

 habits of some forms, tending to injure the quality and growth, or even destroy the 

 life, of this valuable source of food. The larger forms are somewhat rare, compara- 

 tively speaking ; but the smaller forms, especially in the ocean, exist in unnumbered 

 millions. No one who has never drawn a surface net in some sheltered bay has any 

 idea of the myriads of Entomostraca in the sea ; and should this " surface skimming " 

 be perfonned on a still night, the phosphorescence adds not a little to the interest of 

 the occasion. While in tropical seas large numbers of animals produce this light, 

 on the New-England coast the greater part is the result of the Entomostraca, though 

 jelly-fishes and Infusoria furnish their share. 



In the succeeding account each order of Entomostraca is mentioned, but only the 

 most important families are enumerated. 



ORDER L COPEPODA. 



This order embraces many of our commonest fresh-water Entomostraca, as well as 

 very numerous salt-water forms. In number of species, and in economic importance 

 as well, it stands first. In range of habitat, and in reproductive power, the group 

 surpasses all others among the Crustacea. The animals are mostly small, but are 

 very active; better fitted for locomotion and for a predatory life than are the 

 Cladocera, and so find all waters and all localities suited to one or more of their many 

 species. 



