66 
Tanks No. 2, 5, 6, 10 etc. 
resemble the parent form, but the grand-parent. Thus in one species we 
have two forms which alternate with each other (cp. p. 23); one form 
he found always as a single individual, whereas the other one occurred 
in chains consisting of a large number of individuals. Later observations 
have entirely confirmed the accounts of this ''Alternation of generations” 
of the Salpae. In the Aquarium visitors will find both chains (Fig. 114) 
and single individuals (Fig. 113) of the same species; the chains are some¬ 
times of considerable length, or they may form a closed ring (Fig. 115). 
The members of such a chain are completely alike, and are hermaphrodite. 
Their eggs never hatch into chains, but always into single individuals, 
which are not only different anatomically from their parent, but also 
never produce eggs. Instead of these latter their ovary gives rise to 
buds, which are small chains of Salpae and are liberated after attaining 
a certain size. Like Pyrosoma the Salpae are phosphorescent animals, 
most light being given out by the intestinal coil. 
In connection with the Tunicata we may mention the Lancelet, 
Amphioxus laneeolatus (Fig. 116), which is looked upon as the lowest 
Fig. 116. Amphioxus laneeolatus, twice nat. size. 
vertebrate form. Its body is scarcely 2 inches long, colourless and trans¬ 
lucent; its only skeleton is a notochord: it has no head. Instead of 
a heart it has pulsating vessels containing colourless blood, and like the 
Tunicates it has a throat perforated with innumerable gill-slits. But its 
development is even of greater interest to zoologists than its anatomy; 
for it resembles greatly that of the Ascidians (cp. p. 64) and points to 
a near relationship of these two forms. It is supposed that there 
must once have existed a group of animals possessing a notochord and 
gill-slits, whose descendants are now represented by three clans, (1) the 
Fishes and Terrestrial Vertebrates, (2) the lonely Lancelet, (3) the Tunicates. 
Amphioxus lives in the sand of flat coasts and resembles a worm 
more than a fish. It is found in thousands along the beach of Posilipo 
and similar tracts of the Bay of Naples, and has been found in other seas. 
It was first discovered on British coasts by Mr. Couch in 1831 and since 
then has often been found. 
In the Aquarium it can only be kept, if a plentiful supply of sand 
be in the tank (the little open one in front of tank No. 10). In this 
it burrows immediately and only comes out at night or if disturbed. 
FISHES {PISCES). 
(Tanks 2, 5, 6, 10 etc.). 
Fishes have such well-known external characters, that they will be 
rarely mistaken for members of the other large groups of the animal 
