5o8 



UROCHORDA 



surrounded by its own test, but is united to its fellows by 

 a common blood system. In the compound Ascidians, 

 on the other hand, many individuals are enveloped in a 

 common test, and all, like Clavelina, possess the power of 

 reproducing asexually by budding. There is, however, 

 no doubt that the so-called compound Ascidians are an 

 artificial group, whose members diverge widely in structure, 

 though all display the two characters mentioned. 



Some of the compound Ascidians are not fixed, but form 

 floating colonies. These forms lead up to the beautiful 



Fig. 2iS9. — Part of a colony of Boiryllus, showing two individuals 

 embedded in a gelatinous matrix and with a common exhalant 

 aperture. 



C.E.A., Coiumon exhalant aperture ; I. A., inhalant aperture ; B.S., blindly ending 

 blood sinuses of colony ; PH., pharynx ; H., heart ; £., endostyle ; D.L., 

 dorsal lamina ; 6'., stomach ; A'^., nerve ganglion ; PB., peribranchial space. 



Pyrosoma or phosphorescent fire-flame, where the whole 

 colony with its numerous individuals swims as one creature. 

 All these belong to the Ascidian series, and display 

 interesting diversity in their methods of development. 

 The simplest case is that already described for Asa'dia, 

 where the tailed larva gives rise to a sexual adult without 

 any power of budding. This occurs in almost all simple 

 Ascidians, but even here there are indications of possible 

 complication. Thus, on the one hand, in some, e.g. Mol- 

 gula, there is a tendency towards abbreviation — the larval 

 stage being suppressed; while, on the other, the adult 



