The Plathander'' (Xenopus laevis.) 
349 
The dorsal aorta, at the point of its forroation by the two systemic 
trunks and just between the two anterior ends of the kidneys, gives off an 
artery which at once branches into a splenic (Sp) and a mesenteric (Mes). 
Posterior to the above coeliac artery are given olf the renal arteries (R) to 
the kidneys and three pairs of vertebral arteries to the dorsal body-wall. 
Underneath the bladder the dorsal aorta divides into right and left sciatics 
(Sc) ; each sciatic soon divides into two, one of which enters the leg as the 
principal artery of this limb after having given otf a vesicular (Ves) branch 
to the cloaca, bladder, and rectum ; this branch runs along the posterior, 
dorsal face of the hind limb and sends a number of small branches to the 
skin. The second branch of the sciatic breaks up into {a) an iliac (IL) to 
the muscles around the ileum, (b) a cutaneous (Cut) which is a fairly 
large artery to the skin at the side of the body just anterior to the limb, 
(c) an artery to the anterior, dorsal face of the hind-limb (labelled FI in 
Fig. 4), and (d) a small pelvic (Pel) to the posterior end of the pelvic 
girdle. 
It will thus be seen that whereas the skin of the frog is supplied with 
impure blood from the cutaneous branch of the pulmo-cutaneous trunk, 
in Xenopus the skin is supplied with fairly pure (oxygenated) blood from 
the systemic trunk, through the subclavian and sciatic arteries. Also, the 
carotid trunk sends a large branch to the muscular ventral body-wall. 
An interpretation of all this may be found in the fact that the skin of the 
frog acts as a subsidiary respiratory organ, and therefore receives non- 
oxygenated blood, whilst Xenopus, owing perhaps to its life in deep and 
mostly bad water, possesses a skin which has no respiratory function and 
which consequently needs oxygenated blood. The cutaneous branch of 
the pulmo-cutaneous of the Plathander is therefore on its way to becoming 
vestigeal and is already almost wholly replaced by branches of the sub- 
clavian and sciatic arteries. The muscularis of the carotid may also have 
been developed in connection with the degeneration of the cutanea 
magna. 
The Venous System (Fig. 5). — The three caval veins (two prae-cavals 
and a post-caval) and the pulmonary veins are similar to those of the frog. 
The external jugulars (EJ) are, how^ever, very variable, since I have found 
specimens with only the right external jugular, others with only the left, 
still others with both normal, and finally some with the right and left 
joined between the thymus glands as in Fig 5. The skin is mostly 
drained by the cutaneous branch of the innominate vein, but the iliac, 
the femoral, and the brachial also receive branches from the skin. The 
renal portal system is very complicated. The femoral, which is the 
principal vein of the leg, is joined on the one hand to the renal portal 
vein (RPj and on the other to the abdominal (Abd) by means of a short 
thick pelvic (Pelv) ; the pelvic receives cloacal (Clo), cutaneous (Cut), 
