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, ?>/) are outgrowths 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, Ji ) ; in Clado- 

 ceraand 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 antennas, and the shc/t- 

 F glands opening on the bases of the second 



maxilla?. 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 ca-ca 

 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 

 "V.Maja'squmad'o): b ody-cavity like a typical nephridiuiu. 

 thoracic ganglion; eg. Ihe nervous system is alwavs formed 



ganglion ; </. , i i 



on the ordinary arthropod type, as de- 

 scribed in Apus and Astaeus, 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 

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

 immense thoracic ganglion (If/). 



brain ; <, stnmarli ; sir, oeso- 

 phageal cnnneetivtj ; sg, vi*- 

 feral m-rvrs ; n, ]n,M- 680- 

 1'hageal connective, (From 



I'liiiifiHi-ll/ii'i .-1,1111- 



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