464 CRUSTACEA. 



as in the youngest larvae of the Decapoda, only one pair of slits is 

 present and the arterial system has but few branches. In the fully- 

 developed Decapoda the number of paired slits is increased by the 

 addition of a dorsal and a ventral pair, and the vascular system is 

 considerably perfected. An anterior cephalic aorta supplies the 

 brain, the antennae and eyes. Two lateral pairs of arteries send 

 branches to the stomach, liver and generative organs. The posterior 

 abdominal aorta usually divides into a dorsal and a ventral artery, of 

 which the first supplies the muscles of the tail, the latter (known as 

 sternal artery) sends branches to the appendages of the thorax and 

 abdomen (fig. 364). From the ramifications (often capillary-like) the 

 blood flows into larger or smaller canals with connective tissue walls 

 which may be regarded as veins, and from thence into a wide 

 blood space situated at the base of the gills. It thence passes through 



F" 



Fro. 364. Longitudinal section through Aitaca* Jluviatilis (after Huxley). C, Heart; Ac, 

 cephalic aorta; Aa, abdominal aorta, the sternal artery (Sta) is given off close to its 

 origin; Km, masticatory stomach; D, intestine ; Z, liver; T, testis; Yd, vas deferens; 

 Go, genital opening ; G, brain ; N, ganglionic cord ; Sf, lateral plate of the caudal fin. 



the gills and, having become arterial, passes into other vascular 

 tracts (branchial veins containing arterial blood), which conduct it 

 to a receptacle surrounding the heart, the pericardia! sinus : from the 

 latter the blood enters the heart through the slits which are provided 

 with valves. 



The alimentary canal consists of a short oesophagus, a wide saccular 

 crop and an elongated intestine which opens by the anus beneath 

 the median plate (telson) of the caudal fin. The wide crop or 

 masticatory stomach is supported by a firm chitinous framework, to 

 which are affixed several pairs of masticatory plates (derived from 

 thickenings of the chitinous lining). In the Decapoda two round 

 concretions of carbonate of lime (Cray-fish) may be deposited in the 

 walls of the masticatory stomach beneath the chitinous lining ; these 

 ate the so-called " eyes," and are found in the spring and summer. 





