Tunicate. 



tubes project, while in Phallusia (Fig. 29) only their apertures are vi- 

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

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

 current (see p. 53) which enters the mouth and issues from the out- 

 going 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 

 discharged, the intestine being coilled in the 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. 30) and the crimson- or orange-coloured Cynthia 

 (Fig. 28) ; 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 iu 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 indivi- 

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

 Naples. In the Indian ocean recently there have been found specimens 

 of several metres length. 



The life history of the Ascidians in 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 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 Ascidian it 

 gradually decreases . and vanishes entirely when the tapdole 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 

 thnt the ancestors of the Ascidians were probably swimming animals 

 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. 



