64 
Tanks No. 4 and 20. 
to each other at their base by "runners” like strawberry plants; or, as in 
a third group, the Compound Ascidians, a number of individuals are united 
in a common covering and grouped in definite manner. To these last 
belong Diazona (fig. Ill) and the various species of Botryllus, which 
form patches on the rocks of the tank; the arrangement of the individuals 
in the shape of rosettes can in this case be seen with the naked eye. The 
only free-swimming Ascidian known is the Pyrosoma (Fig. 112); a hollow 
gelatinous cylinder from which the separate individuals project like the 
pegs on the cylinder of a musical box. It belongs to the pelagic fauna, 
and helps materially to produce the wonderful phosphorescent appearance 
of the sea. It is only rarely seen in the Aquarium (Tank 20), being 
of irregular occurrence in the Bay of Naples. 
The life history of the Ascidians is extremely interesting. From 
the egg escapes a free-swimming tadpole, with lashing tail, containing 
an organ which at the commencement has great similarity with the 
"notochord” of Vertebrates. The "notochord” is a cartilaginous rod, round 
which the back-bone is formed; in the lowest Vertebrates it persists 
throughout the life of the animal, but in the larval Ascidian it gradually 
decreases, and vanishes entirely when the tadpole becomes fixed. The 
theory has been scientifically established, that every individual in develop¬ 
ing passes through stages, which represent the form of its ancestors; to 
take a simple example: the fish-like form of a frog’s tadpole indicates 
that the ancestors of the frogs were fishes, in other words that the 
frogs have descended from fish-like Vertebrates. Now the young Ascidian 
has a notochord, an eye, and an ear; in other words, it is adapted as a 
swimming animal; we believe therefore that the ancestors of the Ascidians 
were probably swimming forms allied to the Vertebrates, degraded thus 
sadly through the ignominy of a well-protected life. 
All Ascidians are hermaphrodite, i. e. each individual is at once 
male and female. But besides the sexual reproduction, in which fertilized 
egg-cells produce the above-mentioned larvae, asexual reproduction takes 
place by the process of budding, and so gives rise to the colonies. 
Opposed to the sessile Ascidians we have the free-swimming Salpae. 
The transparency of their bodies stamps them as pelagic forms, which 
drift about like the jelly-fish on the open sea, and are occasionally 
