AN UNSETTLED QUESTION 329 



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 peraeoii, 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 peraeopods undeveloped, as 

 .is customary with the Isopoda. 



