17 
for it has no brain, and, hardly any head to speak of ; but as to its 
tail, it ~hows a slight advance on the tadpole ; for it has rudiments 
of fin-rays to support the swimming membrane, and it also 
shows a slight expansion towards the end. The marginal 
fin extends, not only all along the back, but right round 
the snout in front, down to the upper lip. Fig. 3 is a side 
view of the whole animal. 
Now, before proceeding to the true fishes, we may notice those 
most curious creatures known as Ascidians or Tunicates. These, 
in their young state, when just hatched, have the external shape of 
a tadpole. The internal structure and arrangements of the body 
also show some likeness; but in the tail the resemblance is most 
Striking. See Fig. 4. Here we find a distinct notochord, lying 
beneath the nervous axis; with muscles, arranged in segments, 
above and below ; and a vertical fin surrounding all, expanding to 
a fan-shape at the end. This larva swims about like an ordinary 
trog-tadpole. But after a time it fixes itself fast to some surface 
by its underside; and then the tail, no longer needed, gradually 
shrinks, and shrivels up. The nervous system also degenerates ; 
the eye disappears; and the adult animal is evidently far lower in 
rank than the young. But there is one genus of Tunicates, 
called Appendicularia, in which this degradation does not take 
place ; the creature keeps its tail, and swims freely all its life, Is 
is remarkable that these Tunicates spend their whole existence in 
the sea; whereas true tadpoles inhabit fresh water only. 
To come now to higher forms. The Monorrhines, which include 
the Hags and Lampreys, have a simple, primitive type of tail; 
but in the true Fishes there are several curious varieties, not only 
in the different kinds, but even in the same fish at different ages, 
Thus, in most new-hatched fishes the tail is nearly as simple as in 
the tadpole ; but it often has an expansion at the end, reminding 
us of the Lancelet and the Tunicates. 
Another type of fish-tail is seen in the geuus Ceratodus, now 
living in some of the rivers of Australia, but found fossil in the 
ancient Secondary rocks of Europe. See Fig. 5. Here the 
membrane is supported by a number of jointed fin-rays, both 
above and below the notochord. This is called a diphycercal 
arrangement. 
A more common type is seen in sharks and sturgeons, and most 
of the oldest fossil fishes. There are two lobes; the notochord 
extends along the upper lobe, which is generally the longer of the 
two; the lower lobe has only rays. This shows that the upper 
lobe is the true continuation of the axis of the body ; the lower is 
merely part of the marginal fin. This kind is called heterocercal. 
But the most common shape of all is that called homocercal ; where 
