The Mode of Growth of Tubicinella Trachealis. 3 
Darwin does not say by what means the young cirripede forces 
itself into the epidermis of its host, but he describes in a detailed 
way the mode of growth of the parasite settled in the skin, By a 
comparison of specimens of different size, he came to the conclusion 
that the tubular shell grows at its base downwards and in a 
tangential direction, producing in this way the lengthening as well 
as the widening of the shell. As, according to Darwin, the shell is 
sradually pushed out of the skin, its outer edge must soon project 
beyond the level of the skin, and as it is very brittle it continually 
breaks away at the outer edge. If the whole growth of such a shell 
from its infancy, when it is hardly one-fiftieth of an inch in diameter, 
PIECE OF WHALE SKIN WITH SIX TUBICINELLAS (half natural size). 
1. Surface of epidermis. 2. Vertical section through epidermis. 3. Corium. 
until it is of full size, 2.c., about 1 inch in diameter, could be 
preserved and put together, it would form a conical tube at least 
6 inches long. How much longer the cylindrical tube would be 
which one would obtain by putting together the fragments of shells 
breaking away during the life of the adult animal, is, of course, 
impossible to say, as one does not know how long these cirripedes 
live and at what rate the longitudinal growth continues. 
In all its stages the shell is provided with circular ridges which 
make it impossible for it to slip out of the skin. If one assumes 
, that-each ridge remains surrounded by, and in contact with, those 
. 
