22, LIVERPOOL BIOLOGICAL SOCIETY. 
aperture, is of primary importance since it serves the 
following purposes :— 
(1) it conveys oxygen into the body for respiratory 
purposes; 
(2) it brings the food matters into the body; 
(3) it removes waste matters from the body, and 
(4) it conveys to the exterior the ova and spermatozoa. 
This current of water is caused and guided by (a) the 
shape of the mantle and the arrangement of the sphincters 
and other muscles, and (0) the cilia covering certain of the 
vessels and other parts of the wall of the branchial sac. 
Hence modifications of the form of the mantle and of its 
muscles, and of the vessels, bars, papille, &c. forming 
the wall of the branchial sac (which are precisely the 
characters now made use of in describing the species) 
must surely be of functional importance, or in other 
words are useful modifications such as would be produced 
by the action of natural selection. 
It is scarcely necessary to call attention to such 
important adaptive characters as the arrangement of the 
blood vessels and the water passages in the walls of 
the branchial sac, but it may be pointed out that even 
such trivial structures as the spine-lke scales lining the 
branchial siphon in some Cynthiide may well be more or 
less useful according to their shape and size, in keeping 
out small unwelcome intruders, such as the young of the 
parasitic Copepoda sometimes found in the branchial sacs 
of some Ascidians. 
Another point in which species of Ascidians differ is the _ 
condition of the tentacles round the entrance to the 
branchial sac, i.e. their number, shape, branches and ~ 
arrangement. These organs probably perform various 
functions: they break up and distribute the currents of 
water, they intercept and guide the food particles, they 
