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 be 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 egg); 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. 



