M s ELEMENTARY /.OOLOGY 



istics which distinguish them from other Arthropods are 

 the possession of gills for respiration (some insects have 

 gills, but of a very different kind as will be seen later), 

 and the bi-ramose condition of the body appendages, each 

 appendage (excepting the antennules) consisting of a 

 single basal segment from which arise two branches made 

 up of one or more segments. Of the form of the crus- 

 tacean body few generalizations can be made. 



'There is no [other] class in the animal kingdom 

 which presents so wide a range of organization as the 

 Crustacea, or in which the deviations in structure from the 

 4 type form ' are so striking and so interesting from their 

 obvious adaptation to the mode of life. ' ' For this reason 

 no attempt will be made to discuss in general terms the 

 form of ther crustacean body, but brief accounts will be 

 given of a few of the more familiar kinds of Crustacea 

 which will serve to illustrate this remarkable diversity of 

 body form. 



Similarly impossible* is it also to give a general account 

 f the development of the crustaceans. The sexes are 

 distinct in most Crustacea, and there is often great differ- 

 ence in form between the male and female. A certain 

 amount of metamorphosis takes place in the development 

 of all crustaceans; that is, the young when hatched from 

 the egg differs, often decidedly, in appearance and structure 

 from the parent x and in the course of its post-embryonic 

 development undergoes more or less striking change 

 or metamorphosis. This metamorphosis is often very 

 marked. 



Water-fleas (Cyclops). TECHNICAL NOTE. The water- 

 fleas are common in the water of ponds or of 'slow streams; they 

 may often be found in the school aquarium. They are, though 

 small (about i mm. long), readily seen with the unaided eye ; they 

 are white, rather elongate, and have a rapid jerky movement. Ex- 

 amine specimens alive in water in a watch glass. Note the "split 

 pear " shape, broadest near the front, tapering posteriorly, flat be- 



