452 SUB-PHYLUM UROCHORDA OR TUNICATA. 
abundantly represented on British coasts. As contrasted 
with this type, we have in other members of the class most 
remarkable diversity 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. Clavelina), 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 
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 C/avelina 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 dis- 
play 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 diversity 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, eg. Aol 
gula, there is a tendency towards abbreviation—the larval 
stage being suppressed, while, on the other, the adult 
acquires the power of reproducing asexually, eg. C/avelina. 
Both processes are carried further in the compound 
Ascidians. In these the eggs have usually a considerable 
amount of yolk, and development takes place either in the 
atrial cavity of the mother, or in special brood-pouches. In 
consequence, the development, especially in the early stages, 
shows considerable modification, although the larval stage 
is quite distinct. Again, the tailed larva develops into an 
adult which has no sexua! organs, but forms a colony by 
