452 SUB-PHYLUM UROCHORDA OR TUNIC AT A. 



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. 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 

 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 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, 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. 

 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 sexual organs, but forms a colony by 



