lOO ^^''' second. 



individuals are connected the one with the other by processes 

 of the mantle-base. The Compound Ascidians are a third group, 

 in which a number of individuals are united in a common covering 

 and grouped in definite manner. To these last belong Diazona 

 (fig. 31), 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. 

 Pyrosoma is a free-swimming Compound Ascidian (fig. 96), 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 materially helps to produce the wonderful 

 phosphorescent appearance of the sea. It is only rarely seen in 

 the Aquarium (tank Nr. 20), being of irregular occurrence in the 

 Bay of Naples. In the Indian ocean there have been found 

 specimens of several metres in length. 



The life history of the Ascidians is extremely interesting. 

 From the eggs escapes a free-swimming tadpole, with lashing tail, 

 containing an organ which at the commencement has great sim- 

 ilarity with the "notochord" of Vertebrates. The "notochord" 

 is a supporting rod, round which the back-bone is formed; in the 

 lowest Vertebrates it persists throughout the life of the animal, 

 but in the larval Ascidians it gradually decreases, and vanishes 

 entirely when the tadpole becomes fixed. The theory has been 

 scientifically established, that every individual in developing 

 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 x\scidian has a notochord, an eye, and an ear: 

 that is to say: it is adapted to the life of a swimming animal. 

 We believe therefore that the ancestors of the Ascidians were prob- 

 ably swimming animals allied to the Vertebrates, but now sadly 

 degraded through the ignominy of a well-protected life. Allied here 

 is Amphioxus, to be described at the end of this chapter. This 

 also possesses a "notochord", which, however, it retains through- 

 out 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. "Runners", 

 like those of strawberry plants, arise from the base of the mantle, 

 and on these arise buds which grow into new individuals, thus 

 producing the colonies. 



Opposed to the sessile Ascidians we have the free-swimming 

 Salpae. The transparency of their bodies stamps them as pelagic 

 animals which drift about like the jelly-fish on the open sea, 

 and are occasionally carried by currents to the coasts, where they 



