xr PHYLUM ARTHROPODA 549 



tractile heart, formed as a muscular dilatation of a dorsal vessel, 

 and communicating by valvular ostia with an enclosing pericardial 

 sinus. The blood is taken from the heart to the various organs 

 by arteries, and is returned to the pericardial sinus by sinuses 

 and veins : the respiratory organs are interposed in the returning 

 current. The renal organs are peculiarly modified nephridia, 

 which may take the form either of shell-glands opening on the 

 second maxillae, or of antennary (green) glands opening on the 

 antennae. 



The nervous system consists of a brain united by cesophageal 

 connectives with a ventral nerve-cord, formed of a double chain 

 of ganglia joined together by commissures and connectives. The 

 first three pairs of embryonic ganglia commonly unite to form 

 the brain, which is therefore a syn-cerebrum. The sexes are 

 separate or united : sexual dimorphism is common : partheno- 

 genesis frequently occurs. The sperms are either amoeboid with 

 radiating stiff processes or pseudopodia, or flagellate : the 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 very usually has a distinct 

 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 : 



Sab-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 maxillae 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 antennae 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. 



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