OPENING ADDRESS. 21 
the branchial sac, the tentacles, the dorsal lamina, as 
those which yield the most reliable characters. Now, 
these organs have prominent roles to fill in the life of the 
animal in connection with nutrition, respiration, the circu- 
lation of water through the body, and the collection and 
agelutination of food particles, and any changes in their 
size, form and structure must be of considerable physio- 
logical importance. And I find that in going over the 
characters of species of Ascidians, I can recognise a 
marked utility in the various specific modifications of 
these and other organs. Even the very varied external 
shapes may be regarded as useful modifications, since they 
allow of, and correspond to, particular forms of the 
muscular mantle and the branchial sac and the other 
viscera within the test, and of course the shapes of the 
mantle and branchial sac are of great functional impor- 
tance, since they have to do with the animal’s respiration 
and the regulation of the food supply. 
Even in the case of such apparently trivial characters as 
the shapes and distribution of the minute spicules of 
carbonate of lime throughout the colonies of Lepto- 
clinum and some other Compound Ascidians, I know from 
experience that they affect the hardness and roughness 
as well as the colour of the colony, and so may be of 
considerable importance in repelling enemies and in 
keeping the colony free from injurious parasites. As a 
matter of observation I find that the colonies of Didemnidee 
(which are provided with calcareous spicules) are much 
freer from both external and internal parasites than are 
the softer-bodied Compound Ascidians. 
Passing to the interior of the body of the Ascidian we 
find that the current of water pouring in at the branchial 
aperture, through the stigmata in the walls of the branchial 
sac into the atrial cavity, and from that out by the atrial 
