PHYLUM ARTHROPODA 521 



ie eggs are usually centrolecithal but may be telolecithal, or 

 almost alecithal. The muscles are striped, and there are no cilia. 

 Segmentation of the oosperm is usually superficial, but may be 

 complete or discoid. The embryo passes though a nauplius stage, 

 which may be a free-swimming larva or may be passed through 

 before hatching, and is characterised by the presence of three 

 pairs of appendages which become the antennules, antennae, and 

 mandibles of the adult. 



The Crustacea are classified as follows : 



Sub-Class I. Entomostraca, 



Crustacea of comparatively simple organisation, and usually of 

 small, often almost microscopic size. The number of post-cephalic 

 segments is variable, and the appendages show comparatively 

 little differentiation. A large cephalic carapace is often present. 

 A considerable portion of the enteric canal is derived from the 

 mesenteron, and there is no gastric mill. The excretory organs 

 are shell-glands. There is a metamorphosis, and the larva usually 

 leaves the egg as a free-swimming nauplius. 



ORDER 1. PHYLLOPODA. 



Entomostraca in which the body is distinctly segmented and 

 is covered by a cephalic carapace. The post-cephalic appendages 

 are leaf-like. 



Sub- Order a. Euphyllopoda. 



Phyllopoda of considerable size, with 10-60 pairs of leaf-like 

 swimming-feet. 



This group includes Apus and allied genera, such as Branchipus, 

 Artemia, and Estheria (Fig. 413). 



Sub-Order 1). Cladocera. 



Small Phyllopoda with compressed body inclosed in a bivalved 

 carapace, and with four or five pairs of swimming-feet. The chief 

 organs of tocomotion are the large biramous antennae. 

 Including Daplmia, Leptodora, &c. (Fig. 414). 



ORDER 2. OSTRACODA. 



Small Entomostraca having the body inclosed in a carapace or 

 shell formed of articulated right and left valves. The body is 

 unsegmented and the abdomen rudimentary. There are only 

 seven pairs of appendages. The sperms exhibit motility after 

 reaching the female ducts. 



Including Cypris, Cy 'there, &c. (Fig. 415). 



