GENERAL NOTES ON TUNICATA. 405 



single atrial aperture of the adult. The gill-slits increase greatly in 

 number ; the increase being due both to the formation of new slits and to 

 the division of those first formed, and the whole animal undergoes a meta- 

 morphosis which is one of the most signal instances of degeneration. 



General Notes on Tunicata. 



The description of Ascidia given above is, in its general 

 outlines, applicable to all the simple Ascidians, which are 

 abundantly represented on British coasts. As contrasted 

 with this type, we have in other members of the class most 

 remarkable variations in structure, habit, and life history. 



The simple Ascidians are usually sedentary, growing fixed 

 to stones, shells, or weed, and are widely distributed, occur- 

 ring on or near the coasts of all seas. With the exception 

 of the so-called social Ascidians (e.g. Clavelind), they do not 

 reproduce by budding, but are often gregarious, great 

 masses being found together. 



To the compound Ascidians (e.g. Botryllus) those simple 

 forms are linked by Clavelina, where each individual is sur- 

 rounded 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, while all dis- 

 playing the two characters mentioned. 



Some of the compound Ascidians are not fixed, but form 

 floating colonies. These forms lead up to the beautiful 

 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 variations in their methods of development. 

 The simplest case is that already described for Ascidia, 

 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 

 acquires the power of reproducing asexually, e.g. Clavelina. 



