XI PHYLUM ARTHROPODA r,63 



radiating pseudopodia, or flagellate : the eggs are usually centro- 

 lecithal but may be telolecithal, or almost alecithal. The muscles 

 are striped, and there are no cilia. 



Segmentation of the oosi)erm is usually superficial, but may 

 be complete or discoid. The embryo very usually has a distinct 

 nauplius-stage, which may be a Irec-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.— Branchiopoda. 



Crustacea with a varying number of body-segments, provided 

 with appendages of a uniform character, usually foliaceous, rarely 

 leg-like, the posterior region (abdomen) devoid of appendages and 

 provided with a pair of many-jointed or unjointed caudal styles. 

 A cephalic carapace is sometimes absent : when present it may be 

 either shield-like or bivalve. Paired eyes are usually present. 

 The antennules and the maxilhe are reduced or absent : the 

 mandibles devoid of, or with a vestigial, palp. The larva is a 

 nauplius or metanauplius. 



Order. 1. — Anostraca. 



Branchiopoda in which a carapace is not developed. The eyes 

 are stalked : the antennas are prehensile in the male, reduced 

 in the female. The appendages of the body-segments number 

 11 or 19 pairs. The caudal styles are not jointed. 



This order includes Branchipus and Artemia. 



Order 2. — Notostraca. 



Branchiopoda in which there is a large dorsal shield-shaped 

 carapace. The eyes are sessile. The antennae are reduced. There 

 are 40 to 63 pairs of trunk appendages. The caudal styles are 

 many-jointed. 



Including A2nis and Lepidurus. 



Order 3. — Conchostraca. 



Branchiopoda with a carapace divided into two lateral portions 

 or valves like the shell of a bivalve mollusc, and enclosing the 

 entire animal. The antennte are biramous and are used as 

 swimming appendages. The eyes are sessile, coalescent. The 



