596 



ZOOLOGY 



SECT. 



functions as a true lung. Probably the inner surface of the gill- 

 cover or branchiostegite performs a respiratory function in the 

 Crayfishes. 



In Amphipoda, also, the gills (Fig. 471, br) are outgrowths of 

 the thoracic limbs : in Isopods they are the modified endopodites 

 of the second to the fifth pleopods ; in some of the terrestrial 

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

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

 470, 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, 

 Leptostraca, Stomatopoda, Anaspidacea, Tanaidacea, Isopoda, and 

 Amphipoda (Fig. 471, h} ; 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 meson ephridia are present the antennary glands opening 

 on the bases of the antennae, and the maxil- 

 lary or shell-glands opening on the bases of 

 the second maxillae. But as development 

 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 Stoma- 

 topoda, 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 a similar function is assigned to 

 caeca opening into the posterior end of the 

 mesenteron. In some of the Cirripedia the 

 maxillary gland is described as opening into 

 one of the compartments of the body-cavity 

 like a typical nephridium. 



The nervous system is always formed 

 on the ordinary arthropod type, as de- 

 scribed in Apus and Astacus, and the chief 

 variations it presents are connected with 

 the greater or less amount of concrescence 

 of ganglia. In the sessile Barnacles and in the Crabs (Fig. 473) 

 this process reaches its limit, the whole ventral nerve-cord being 

 represented by a single immense thoracic ganglion (bg). 



FIG. 473. Nervous system of a 

 Crab (Maja squinado). 



bfi, thoracic ganglion ; eg. 

 cbmmissural ganglion ; g, 

 brain ; m, "stomach " ; sc, 

 ossophageal connective ; sg, 

 visceral nerves ; y, post-ceso- 

 phageal connective. (From 

 Lang's Comparative Anat- 

 omy, after Milne-Edwards.) 



