TUNICATES 167 
ear-like organ; and the remarkable fact is that the retina and lens 
of the eye and the sensory part of the “ear” are derived from the 
walls of the brain as is the case in vertebrates. 
After swimming about for some time the tunicate tadpole usu- 
ally settles down upon the bottom and fastens itself to a situation 
from which it never departs. Under these conditions it finds 
almost no need for sense organs or skeleton, and accordingly these 
degenerate to such a degree that the eye disappears completely, 
and the other structures are reduced to mere remnants. 
But all tunicates do not thus settle down and degenerate, for 
some of them remain active throughout life, swimming rapidly 
through the water. One of these free forms called Appendicularia 
remains tadpole-like in shape throughout its existence, having a 
pair of gill-slits, one on each side, and a long powerful tail which is 
provided with a fin, and arises from the middle of the ventral side 
of the body. 
Many tunicates are solitary animals while others produce 
large colonies by budding, the older members of the colony giving 
rise to the younger. In other forms such as Salpa, there is an alter- 
nation of generations, one being produced from eggs and the fol- 
lowing generation through budding. 
A tunicate has been aptly compared to a leather bottle with 
two spouts. The outer covering of the body is usually tough 
in consistency and contains cellulose, the composition of which is 
identical with the substance that forms the walls of plant cells. 
A moment’s observation of the two funnel-shaped spouts will 
show that water is constantly being drawn into one and forced out 
from the other, and a further study shows that the water is drawn 
in at the spout at the front end of the body, and passed out of the 
opening upon the back of the animal. The intestine is U-shaped, 
and the mouth is at the place where the water enters while the vent 
is at the spout through which the water passes out. 
The throat is a wide sac almost as long as the body itself, and 
is pierced by so many little gill slits that its sides resemble a sieve. 
The water enters the mouth, passes through these gill slits, and 
finally out through the dorsal spout; the current being maintained 
by the beating in unison of thousands of hair-like cilia which line 
the gill slits. 
