XI 



PHYLUM ARTHROPODA 



581 



forms, in adaptation to aerial respiration, a system of air-tubes is 

 developed in the exopodites ; in Stomatopoda, gill-filaments (Fig. 

 477, br) spring from the exopodites of the first to the fifth pleopods. 

 Moreover many Crustacea perform rhythmical contractions of the 

 intestine, taking in and expelling water : such anal respiration 

 is common among the lower groups, and is especially noticeable in 

 Cyclops. 



The heart is absent in many Copepods (including Cyclops), in 

 some Ostracoda (including Cypris), and in Cirripedia : it is an 

 elongated tube with several pairs of ostia in Euphyllopoda, Lepto- 

 straca, Stomatopoda, Anaspidacea, Tanaidacea, Isopoda, and 

 Amphrpoda (Fig. 478, li) ; in Cladocera and Decapoda it is shortened 

 to an ovoid sac with one or more pairs of ostia. 



Excretory Organs. In many larval Crustacea two pairs of 

 modified mesonephridia are present the antennary glands opening 

 on the bases of the antenna?, and the 

 maxillary or shell-glands opening on the 

 bases of the second maxilla?. But as deve- 

 lopment proceeds one pair nearly always 

 atrophies, the maxillary gland alone being 

 usually retained in the Branchiopoda, 

 Ostracoda, Copepoda and Cirripedia, the 

 antennary gland in the Malacostraca. In 

 the Stomatopoda, however, there is no 

 antennary gland, and the function of renal 

 excretion may be discharged by a pair of 

 glandular tubes opening into the rectum ; 

 and in Amphipoda, though antennary 

 glands are present, an excretory function 

 is also assigned to a ca3cum or a pair of 

 cseca opening into the posterior end of the 

 mesenteron. In some of the Cirripedia the 

 maxillary gland is described as opening into 

 of the compartments of the body- 



one 



FIG. 480. Nervous system of a 

 Crab (Maia squinado). 



bg. thoracic ganglion ; eg, 

 commissural ganglion ; g. 

 brain ; in. gizzard ; se. 

 O3sophageal connective ; sg. 

 visceral nerves ; y. post-ceso- 

 phageal connective. (From 

 Lang's Comparative Anat- 

 omy, after Milne-Edwards.) 



cavity like a typical nephridium. 



The nervous system is always formed 

 on the ordinary arthropod type, as described 

 in Apus and Astacus, and the chief varia- 

 tions it presents are connected with the greater or less amount of 

 concrescence of ganglia. In the sessile Barnacles and in the Crabs 

 (Fig. 480) this process reaches its limit, the whole ventral nerve-cord 

 being represented by a single immense thoracic ganglion (bg). 



The sense-organs are mostly of the same character as those of 

 the two examples. The median or nauplius-eye always occurs in 

 the larva, and can frequently be shown to exist in the adult of 

 even the higher groups (Decapoda). The Cirripedia and many 

 parasitic Copepods are eyeless in the adult, as also are certain 



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