362 



COMPARATIVE AX ATOM!' 



CHAP. V 



It follows, from the above description, that in the Ampliipoda the arterial blood 

 cannot in any way be sharply distinguished from the venous blood. 



Anisopoda. This division of the 

 Arthrostraca, though in many points 

 approaching more nearly the organ- 

 isation of the Isopoda than that of 

 the A'niphipoda, shows in its blood- 

 vascular system greater similarity to 

 the latter. Two abdominal aorta?, 

 however, arise from the posterior end 

 of the thoracic heart, and the peri- 

 cardial sinus is continued into the 

 abdomen. The heart of Apscudcs 

 possesses two ostia on the left side 

 and only one on the right ; in youth 

 the 2 pairs of ostia are present. As 

 to the significance of this fact see 

 below (p. 367). 



In all Arthrostraca the paired 

 ostia, as well as the points of origin 

 of the aorta are provided with valves. 



Thoraeostraea. The cir- 

 culatory system here is linked 

 on to that of the Isopoda. Start- 

 ing with the Stomatopoda, the 

 circulatory system of the older 

 larvte of Squill a, known as 

 Alima and Erichthus, which can 

 hardly be distinguished from 

 that of the adult, have been 

 the most carefully investigated. 

 The heart (Fig. 243) extends 

 as a many -chambered dorsal 

 vessel from the maxillar region 

 (behind the stomach) through 

 the thorax and the abdomen to 

 the end of the 5th abdominal 

 segment. Two divisions can 

 be distinguished in it, a short, 

 wide anterior, and a long pos- 

 terior division. Probably the 

 anterior division alone eor- 



FIG. 242. Diagram of the circulatory system of 

 the Ampliipoda, from the side (after Delage). Most of 

 the lettering as in Fig. 240. pt, Pericardial vessels 

 rising from the epimeres (ep), extremities (lirf), and 

 gills (6)-) of the thorax ; pa, pericardial vessels of the 

 abdomen ; s, ventral sinus ; ep, epilneres. The epi- 

 meres of the thoracic segments IV and V partly cut 

 off; 2)1, pleopoda. 



FIG. 243. Circulatory system of an older Squilla larva before it has passed into the Squilta 

 form (after Glaus), h, Heart, continued posteriorly into the many - chambered dorsal vessel 

 which is richly provided with pairs of ostia (o) ; ac, cephalic aorta ; oo, optic artery ; oi, o 2 , 

 arteries of the two pairs of antenna ; am, marginal artery of the dorsal shell ; ast, arteria sternalis ; 

 al, hepatic artery ; as, shell artery ; BJ, 1st lateral artery of the dorsal vessel ; aa&x to aa& 6 , lateral 

 arteries of the abdomen; dr, glandular saccules on the hind-gut; I, hepatic lobes in the telson ; 

 p K , 6th pleopod (uropod) ; tt>, branchial leaves (epipodial appendages of the oral feet). 



