550 



ZOOLOGY 



SECT. 



part of the gill-chamber is separated from the rest, and forms an 

 almost closed cavity into which vascular tufts project : it thus 

 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. 435, &r)^a-etttgrowths of the 

 thoracic limbs : in Isopods they are the modified endopodites of the 

 second to the fifth pleopods: in Stomatopoda, gill-filaments (Fig. 

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

 Moreover many Crustacea perform rythmical contractions of the 

 intestine, taking in and expelling water: such anal respiration 

 is common among Entomostraca, 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, and Arthrostraca (Fig. 435, h) ; in Clado- 

 cera 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 meso-nephridia are present, the antennary glands opening 

 on the bases of the antennae, and the sheU- 

 glands opening on the bases of the second 

 maxillae. But as development proceeds 

 one pair always atrophies, the shell-gland 

 alone being usually retained in the Ento- 

 mostraca, the antennary gland in the 

 Malacostraca. In the Stomatopoda, how- 

 ever, there is no antennary gland, and the 

 function of renal excretion may be dis- 

 charged by a pair of glandular tubes open- 

 ing 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 shell-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 con- 

 crescence of ganglia. In the sessile 

 Barnacles and in the Crabs (Fig. 437) this process reaches its 

 limit, the whole ventral nerve-cord being represented by a single 

 immense thoracic ganglion (by). 



Fi<;. 437. Nervous system of a 

 Crab (Maja squinado). 



lit/, thoracic ganglion ; c<j. 

 c'onmiissimil ganglion; ;/, 

 brain ; //<, stomach ; *<, oaso- 

 phageal connective ; *(/, vis- 

 coral nerves ; ?/, post-oeso- 

 phageal connective. (From 

 Lang's ('i>nifiii,-(il!i-< Anat- 

 Milne-Edwards.) 



