378 ARTHROPODA 



gill chamber being modified into a kind of lung traversed by numerous 

 blood-vessels. In the palm crab (fig. 399) the gill chamber is divided 

 into two portions, the upper part being pulmonary, the lower containing 

 the reduced gills. 



Correlated to this localized respiration is the nearly closed circulatory 

 system (fig. 400, A, B). The heart (//), a compact pentagonal organ, 

 receives its blood from the pericardial sinus (pc) through three pairs of 

 ostia, and forces it out through five arteries to the capillary regions of the 

 body. The venous blood collects in a large venous sinus at the base 

 of the gills (r), passes thence through gills, and is returned by several 

 branchial veins (vbr) to the pericardium. 



FIG. 398. Gills of Astacus exposed by cutting away the branchiostegite. pdb, plb, 

 podo- and pleurobranchia of the corresponding segments; r, rostrum; i, stalked eyes; 

 2, 3, antennae; 4-6, mandibles and maxillae; 7-9, maxillipeds; 10, 14, bases of thoracic 

 eet; 15, first pleopod. 



The alimentary canal is straight and has only one conspicuous enlarge- 

 ment, the so-called stomach (fig. 400, A, m), divided into two portions, 

 an anterior (cardiac) sac, lined with chitinous folds and teeth and serving 

 to chew the food and bearing in its walls the 'crab-stones,' masses of 

 calcic carbonate stored up to harden the armor rapidly after the molt. 

 The second (pyloric) portion of the stomach is guarded by hairs and 

 serves as a strainer, allowing only food sufficiently comminuted to pass. 

 The two liver lobes voluminous masses of branched glandular tubes 

 (/) open just behind the stomach. 



The two antennal glands (C, gd), each provided with a large urinary 

 bladder (&/), are dirty green in color, whence the name green glands often 

 given them. The gonads (fig. 401) lie close beneath the heart, those of 

 the two sides being united behind, while their ducts remain separate. 

 The structure of the nervous system is in part dependent upon that of the 

 abdomen. In the Macrura (fig. 400, C) the ventral chain consists of 

 six ganglia in the thorax, six in the abdomen, but in the Brachyura 



