H. J. Clark on Polarity and Polycephalism. 71 
to the former as supply does to demand, as nutrition does to the 
Antero-posteriority exhibits the same interchangeability as 
bilaterality, but, although plainly enough, not so conspicuously 
in a comparative, homological sense as in the physiological 
interplay of the functions, such as we see in the relations of the 
allantois to respiration in the embryo, or in the ratio of excretion 
of the renal organs dependent upon the degree of activity of 
- ‘és 4 - o ®, 0 Fao ds : 
laria, or in the singularly stellate disposition of the zooi 
of Botryllus, with their common cloacal orifice. 
