16 BRITISH COPEPODA. 



and other purposes ; and respiration is usually carried 

 on by means of branchiae or gills. There is always an 

 external skeleton or " crust " composed of a hard 

 calcareous or a more flexible " chitinous " material, 

 from which investment the class derives its name. 

 All, or almost all, the Crustacea pass through a series 

 of metamorphic changes before reaching maturity. 



The Crustacea are divided into several sub-classes, 

 the most important of which are the CIRRIPEDIA or 

 Barnacles ; MALACOSTRACA, including such animals as 

 crabs, lobsters, shrimps, sandhoppers, and woodlice; 

 and the ENTOMOSTRACA, one order of which forms the 

 subject of this monograph. The Entomostraca are 

 mostly very minute animals, the vast majority of living 

 species varying between one fifth and one fiftieth of an 

 inch in length. Some, however, are much larger, reach- 

 ing a length of an inch or an inch and a half. The Ento- 

 mostraca are somewhat difficult to define in a way at 

 once accurate and characteristic. The following is 

 Professor Huxley's statement : " In the Entomo- 

 straca, if the body possesses an abdomen (reckoning 

 as such the somites which lie behind the genital aper- 

 ture), its somites are devoid of appendages. Moreover, 

 the somites, counting that which bears the eyes as 

 the first, are more or fewer than twenty. There are 

 never more than three pairs of gnathites. The embryo 

 almost always leaves the egg in the condition of a 

 Nauplius ; that is, an oval body, provided with two 

 or three pairs 'of appendages, which become converted 

 into antennary organs and gnathites in the adult. 

 This division of the Entomostraca comprises the 



