360 



COMPARATIVE ANATOMY 



CHAP. 



surface of the shell (which is kept clean by long maxillar feelers), and in 

 the lamellate exo- and epipodites of the thoracic feet. In these parts 

 a brisk circulation takes place. The blood which flows through the 



shell comes from the anterior aorta 

 and re-enters the heart by the dor- 

 sally placed ostia. 



Arthrostraea. First of all we 

 must here point out an important 

 difference in the position of the 

 heart in the two principal divisions 

 of the Arthrostraea, the Isopoda and 

 Amphipoda. In the Isopoda by far 

 the largest portion of the heart, 

 which is provided with 1 to 2 pairs 

 of ostia, lies in the abdomen ; in 

 the Amphipoda, however, the tubu- 

 lar elongated heart which is almost 

 everywhere provided with 3 pairs 

 of ostia lies in the thorax. This 

 difference may be explained by 

 supposing that the Isopoda have 

 retained only the abdominal por- 

 tion, and the Amphipoda only the 

 thoracic portion of the primitive 

 Malacostracan heart which ran 

 through nearly the whole length 

 of the body, and was provided 

 with many pairs of ostia. The 

 localisation of the respiration has 

 probably played the chief part in 

 bringing about this differentiation, 

 since in the Isopoda respiration 

 takes place in the rami, and prin- 

 cipally in the endopodites of the 



abdominal feet (pleopoda), but in 

 the Amphipoda chiefly in the pouch- 

 like branchial appendages of the 

 thoracic feet. 



In the Anisopoda the he. art lies 

 as in the Amphipoda in the thorax. 



.lr 



O.CL 



FIG. 240. Diagram of the circulatory system of the Isopoda, seen from the side. The right 

 thoracic and cephalic walls removed. A part of the intestine (d) cut away (after Delage). Arterial 

 system red, venous system blue, nervous system black. ai, Anterior; an*, posterior antemue ; 

 c(b, cephalo-thorax ; II-V1II, 7 free thoracic segments; u-^-a-j, V abdominal segments; br, gills 

 (pleopoda) ; g, brain ; rl, intestine ; A, heart ; o, ostium of the heart ; pc, pericardium ; mi, anterior 

 aorta ; la, lateral arteries ; t, thoracic arteries ; ha, hepatic artery ; si, lateral sinuses of the thoracic 

 region ; sa, abdominal sinus ; abg, points of insertion of the thoracic feet ; pg, subneural vessel (the 

 dotted line should stop at the red line); ~bp, branchio-pericarclial vessels ; zg, vessels leading to 

 the gills ; are, abdominal aorta ; os, ostia (?) of the lateral sinuses. 



