17 



for it lias no brain, and, hardly any head to speak of ; but as to its 

 tail, it r-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 Firj. 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 Apjwndicularia, in which this degradation does not take 

 place ; the creature keeps its tail, and swims freely all its life, Ic 

 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 illonorrJiines, 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 genus Ceratodus, now 

 living in some of the rivers of Australia, but found fossil in the 

 ancient Secondary rocks of Europe. See /'Vo-. 5. Here the 

 membrane is supported by a number of jointed fin-rays, both 

 above and below the notochord. This is called a diphy cereal 

 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 hotnocercal ; where 



