AN UNSETTLED QUESTION 829 
most important argument for the proposed alliance between 
them. But the position of the heart, when considered in 
connection with the branchial arrangements, points rather 
in the direction of the Cumacea than the Amphipoda, and 
although in the Isopoda the heart is usually in the pleon, 
there are various genera in which it extends more or less 
into the perzon, thus establishing a gradation to the 
Tanaids in which it is wholly there. From the point of 
view of embryology, Bate and Westwood say of Tanais and 
Apseudes that ‘the development is after the manner of the 
Amphipoda rather than that of the Isopoda,’ but in point 
of fact, though the curvature of the embryo in the egg 
_is inward as in the Amphipoda, the young leave the 
mother’s pouch with the last pereeopods undeveloped, as 
is customary with the Isopoda. 
