CRUSTACEA BLOOD-VASCULAR SYSTEM 



361 



The following is a rather more detailed description of the circula- 

 tion of the Arthrostraca. 



Isopoda (Figs. 240 and 241). The heart, which lies for the greater part in the 

 abdomen, and is provided with 1 to 2 pairs of lateral ostia, is closed blindly behind. 

 Out of it 11 arteries arise, viz. (a) a medio-dorsal thoracic aorta running to the head 

 and the eyes, and supplying the cerebral ganglia, and the 2 pairs of antennae, (&) one 

 pair of lateral arteries for the anterior 

 thoracic segments, and the posterior 

 cephalic region together with the ex- 

 tremities of these regions, (c) 3 pairs 

 of thoracic arteries for the 3 posterior 

 thoracic segments and their extremi- 

 ties, (d) 1 pair of abdominal aortse for 

 the abdomen and its limbs, which 

 function as gills. The thoracic aorta 

 forms anteriorly in front of the brain 

 a ring which embraces the oesophagus, 

 from which a subneural artery runs 

 longitudinally under the ventral chord 

 the whole length of the body. This 

 also gives off branches to the limbs. 

 Besides the blood lacunae which lie 

 between the viscera, there is generally 

 in the thorax a large, paired, ventral 

 blood sinus, which in the abdomen FIG. 241. Conilera cylindracea (after Delage). 

 becomes unpaired. 5 pairs of vessels Transverse section of the abdomen. Most of the 



conduct the venous blood out of the letters have the same meanin S as in Fi S- 24 - & > 



, . . , n Heart : x and y, muscle layers (muscle lamella) for 



abdominal sinus into the pleopoda movin g the gil S (pieopoda)! flexor and extensor ;,, 



which function as gills. 5 pairs of is laid back . aTO> the abdominal arteries which sup- 



efferent vessels (veins) conduct the ply them ; ex, outer ; en, inner branchial lamella of the 



blood which has become arterial in pleopoda (exo- and endopodites) ; ag, efferent ; zg, 



*"*'' "' 



the giUs into the pedcarfial sinus; 



it passes thence through the ostia 



into the heart, and by the contraction of this latter it is again dispersed through the 



arteries. 



Amphipoda (Fig. 242). The Amphipoda present a striking contrast to the Iso- 

 poda in that in the latter the vascular system, in the former the lacunar system, is 

 the most pronounced. The long tubular heart which generally lies in the 5 or 6 

 anterior free thoracic segments, usually possesses 3 pairs of ostia, less frequently one 

 (Corophium} or 2 (Platyscelidce). It is continued into an anterior and a posterior 

 medio-dorsal aorta, which pour the blood either direct or through further arterial 

 branches into a large ventral sinus which lies between the integument and the 

 intestine and runs through the whole length of the body. Special afferent blood 

 streams (vessels ?) conduct the mixed blood into the extremities of the thorax and 

 abdomen, thus also into the branchial pouches of the thoracic limbs. Special 

 efferent streams collect the blood in these extremities (thus also the blood which has 

 become arterial in the gills) and conduct it back through 7 vascular loops in the 

 thorax and through 6 in the abdomen into the pericardium, which stretches back 

 beyond the heart to the end of the abdomen. In Corophium the abdominal portion 

 of the pericardium and the abdominal vascular loops are wanting. The blood- 

 vascular system of the Caprellidce agrees in the main with that of other Amphipoda, 

 allowance being made for the reduction of the abdomen. 



