Tunicata. 85 



of these tubes project, while in Phallusia Fig. 29) only their apertures 

 are visible, the thick knobbed tunic giving an irregular outline. Food and 

 oxygen are obtained in all the group by means of an almost continuous 

 current 'op. 53} which enters the mouth and issues from the outgoing 

 pore, passing from the one to the other through innumerable gill-slits in 

 the throat, on the walls of which vibrating hairs drive the water along; 

 an outgoing chamber surrounding the throat communicates with the 

 outgoing pore. Into this chamber the eggs and excrementa are also dis- 

 charged, the intestine being coiled in the comparatively small solid part 

 of the body. 



The Ascidians are nearly all sessile animals; they either remain 

 separate individuals like the above described Phallusia, the semitrans- 

 parent Ciona (Fig. 28} and the crimson- or orange-coloured Cynthia 

 Fig. 30); or they form colonies in which the individuals are connected 

 with 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. 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. The only free-swimming Ascidian known is the Pyrosoma 

 Fig. 96 ; a hollow gelatinous cylinder from which the separate indi- 

 viduals project like the pegs on the cylinder of a musical box. It 

 belongs to the pelagic fauna, and helps materially to produce the won- 

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

 the Aquarium Tank Nr. 20), being of irregular occurrence in the Bay 

 of Xaples. 



The life history of the Ascidians is extremely interesting. From 

 the egg escapes a free-swimming tadpole, with lashing tailj 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 indivi- 

 dual 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 Ascidian has a notochord, an eye, and an ear: in other words, 

 it is adapted to the life of 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 Salpce. 



