THE ISOPODA 209 



system in the Isopock are the great development ami minute rami- 

 fication of the arterial vessels in the more specialised types and the 

 posterior situation of the heart, the latter feature being correlated 

 with the localisation of respiration in the abdominal appendages. 



The heart lies at the junction of thorax and abdomen, extending 

 for varying distances into each. When elongated and tubular it 

 may lie for the greater part of its length in the thorax (Jaera) or 

 be produced backwards into the abdomen. When shortened and 

 saccular it lies mainly in the abdomen, only extending into the 

 last thoracic somite. It communicates with the well-defined peri- 

 cardium l)y one or two pairs of ostia. 



Anteriorly the heart gives off the median aorta and seven pairs 

 of lateral thoracic arteries. Of these the three posterior usually 

 originate separately from the heart, the remaining four arising 

 from a common trunk on each side. Rarely all seven pairs spring 

 laterally from the aorta. Anteriorly the aorta passes behind and 

 below the l)rain to form, except in certain degraded or parasitic 

 forms, an oesophageal ring encircling the gullet in front of or below 

 the oesophageal nerve- ring. Posteriorly this ring is connected 

 with a ventral system of vessels which vary considerably in their 

 arrangement. In the more typical cases (Cymothoidae, Sphaero- 

 midae) a large median subneurul artery runs backwards from the 

 oesophageal ring along the sternal surface of thorax and abdomen, 

 fiiviny; off numerous lateral branches. The lateral thoracic 

 arteries mentioned above also contribute to the ventral system, 

 each one bifurcating as it reaches the lateral part of the 

 corresponding somite, one branch passing into the limb and 

 the other ramifying towards the middle line. Between the 

 ultimate ramifications of these two sets of vessels, those, namely, 

 of the subneural and of the ventro-lateral arteries respectively, 

 anastomoses frequently occur, and in this way a communication 

 is established in each somite between the dorsal and the ventral 

 arterial systems. But, in addition, in one or other of the thoracic 

 somites it is found that the ventro-lateral artery on each side passes 

 directly into the subneural vessel, thus establishing a complete 

 arterial circle. In certain forms {Idotea, Ligia) the subneural 

 artery is for the most part absent, and the ventral system is 

 iormed almost entirely by the ramifications of the ventro-lateral 

 arteries. Vestiges of the subneural artery, however, persist 

 anteriorly where a short trunk runs backwards for a little way 

 from the oesophageal ring, and posteriorly w^here an abdominal 

 trunk is formed by anastomosis of branches from the last pair of 

 ventro-lateral arteries, while in the thorax similar anastomoses give 

 rise to a succession of short vessels in the middle line as though the 

 subneural vessel had become disintegrated into sections. 



The posterior end of the heart always ends blindly and , is never 



14 



