220 INVERTEBRATE ZOOLOGY 



branch from the posterior vessel soon after the latter leaves the 

 heart. Blood is received into the heart from the pericardial sinus 

 through openings in the heart wall called ostia. The capillaries 

 of the arteries allow the blood to pass into a large sternal sinus. 

 In the thorax, offshoots from the sternal sinus pass into each gill 

 as an afferent branchial vessel or vein. Within the branches of 

 the gills, each afferent vein communicates with an efferent 

 branchial vein from which the blood is returned to the peri- 

 cardial sinus. 



Digestive System. — From the mouth opening between the 

 mandibles on the ventral surface of the head, the digestive 

 system passes as a short tube, the esophagus, into a spacious 

 chamber termed the stomach. The latter is divided into two 

 regions: a large anterior sac, the cardiac chamber, which bears 

 a chitinous organ for grinding food; and a smaller posterior 

 pyloric chamber. The intestine, which proceeds from the 

 pyloric chamber, receives the ducts from the two lateral hepato- 

 pancreases. On the wall of the cardiac chamber, there are fre- 

 quently hard, rounded masses of lime, the gastroliths. 



Excretory Organs. — Within the head, near the base of each 

 antenna, lie the excretory organs, the green glands. Each 

 discharges to the exterior by a pore located on the base of the 

 antenna. 



The central nervous system consists of a ventral chain of gan- 

 glia the number and disposition of which are correlated with the 

 extent of the development of the abdomen. A brain, near the 

 anterior extremity of the body, communicates with the ventral 

 chain of ganglia by means of a pair of circumesophageal connec- 

 tives. Six thoracic and six abdominal ganglia comprise the chain, 

 except in the crabs, which have a rudimentary abdomen and here 

 the entire ventral chain becomes fused to form a single ganglionic 

 mass. 



Reproduction. — The gonads frequently consist of two lateral 

 and a single median lobe from which a pair of ducts lead to the 

 genital openings on the ventral surface of certain walking legs. 

 The eggs undergo superficial cleavage and give rise to either a 

 larva or a young adult the larval stages of which are all passed 

 through while in the egg. Considering the relative uniformity 

 in structure of the adult decapods, there is a surprisingly great 

 number of larval forms. Typically, the larva which leaves 

 the egg is a zoea. From this develops a mysis stage (Fig. 95 A) 



