84 ASCIDIANS CHAP. 
The rudiment of the bud is in typical cases composed of two 
vesicles, an outer derived from the ectoderm of the parent and 
enclosing free blood-cells (mesodermal) between its wall and that 
of the inner vesicle—which is usually of endodermal origin, but 
in Botryllidae is derived from the peribranchial sac, an ecto- 
dermal structure. The inner vesicle, derived in the two cases 
from different germ-layers, forms the same organs of the bud, 
and these organs may lw of widely different origin in the- 
larva. Moreover, free cells of the blood may play in the bud a 
very important part, and give rise (Perophora) to such important 
systems as pericardium and heart, neural tube and ganglion, the 
gonads and their ducts, some of which are of ectodermal and 
others of endodermal origin in the larva. 
In some cases of precocious budding (blastogenetic accelera- 
tion) the young buds begin to appear during the tailed larval 
stage. The larva may even contain a first blastozooid (bud) 
with a branchial sac as large as that of the oozooid (derived from 
the ege); and in the Diplosomatidae the larva (see Fig. 42, F), 
when it settles down, may be already a small colony of three 
young ascidiozooids. 
The larvae in most Compound Ascidians, in place of adhering 
papillae, have several or even a considerable number of ecto- 
dermal tubes or prolongations from the body (see Fig. 42, E and 
F) into the surrounding test. These apparently aid in the 
formation of the common test of the young colony, which grows 
over and adheres to foreign objects. 
There are many irregularities in the larval development of 
Compound Ascidians, due to the very different amount of food-yolk 
present in the ova in different genera. In some cases there is even 
dimorphism, two forms of larvae being found in the same colony. 
Compound Ascidians are amongst the most varied and 
brilliant of sessile animals seen at low tide on our own and most 
other coasts. Some are stalked and form club-shaped or knob- 
like outgrowths. Others again form flat gelatinous expansions 
attached to sea-weeds or stones, and are symmetrically marked 
with bright spots of colour in the form of circles, meandering 
lines, or star-like patterns. In such colonies each spot of colour 
or ray of a star represents an ascidiozooid or member of the 
colony, equivalent to the whole animal in the case of the solitary 
Simple Ascidian. 
