260 
A THREE-EYED REPTILE. 
into a cephalic lobe, which may be described as the brain; 
hence we have a brain and spinal cord as in man. From the 
fore-part of the brain a single eye and a single auditory 
organ are developed. 
Contemporaneously with these changes a mouth, oeso¬ 
phagus, stomach, and intestine have been formed, while on 
either side of the body, on a level with the brain, two 
branchial sacs or gills have been developed. As in a fish, 
water is taken into the mouth and driven out through the 
gills. 
Below the mouth three papillae grow out, and from the 
outer layer of the body a cuticular test is deposited. 
The larva is at first confined within the egg-membrane, 
but soon escapes and swims about, now much resembling a 
tadpole. But unlike the tadpole, instead of developing into 
a higher form, it retrogrades, becomes fixed to a stone, loses 
its tail and the greater part of its nervous system, and when 
adult is the degraded form we started with—the Sea-squirt. 
In this stage, the gills open into an atrial chamber which has 
an aperture near the mouth, and it is through this that water 
is ejected when the animal is irritated. It is interesting to 
note that there is one Ascidian— Appendicularia —which does 
not retrograde in development but keeps the tadpole form 
throughout life. 
It is on account of the presence of the following 
structures in both Ascidia and Vertebrata that they are 
classed together 
1 . There is a notochord which lies between the nervous 
system and the alimentary canal. 
2 . The nerve-cord is dorsal and not ventral as in 
worms, insects, molluscs, &c. 
8 . Gill-slits are present which perforate the walls of the 
pharynx. 
From this and other evidence it has been conjectured that 
the ancestral forms of the back-boned animals had a notochord 
as their sole axial skeleton ; a ventral mouth surrounded with 
suctorial structures ; and very numerous gill-slits. It must 
not, however, be supposed that the Ascidians are direct 
ancestors of the Vertebrates ; on the contrary, they are only 
degenerate off-shoots from the ancestral stock. 
Now, although the Ascidian larva closely resembles the 
Vertebrates in the most important points, yet in several 
details it differs, and most obviously in possessing a single 
median eye instead of two lateral eyes. This eye is developed 
as an outgrowth from the brain, and in that respect resembles 
the vertebrate eye, but it is very much simpler in structure, and, 
