XVII 



PROTOCHORDATA 



641 



parent, except that they have a different number of muscular bands. 

 It is uncertain whether these median buds, termed phorozooids, 

 break loose and develop into individuals with sexual organs, or 

 whether they produce ventral stolons from which the true sexual 

 generation arise as buds. 



Asc 1 



phos 



at- 



FIG. 466. Primordial colony of Pyrosoma giganteuvi consisting of four Ascidiozooids 

 coiled round the degeneration Cyathozooid. (After Julin. ) 



The buds of Doliolum are in so far peculiar that the inner vesicle 

 is represented by a solid mass of cells, in which strings of cells are 

 gradually differentiated which develop into pharynx, atrial cavities, 

 muscular tissues, and nervous system. 



In Botryllus and its allies the buds arise as two layered vesicles, 

 and their development is exactly what has been described in the case 

 of Distaplia. But whilst the outer wall of the vesicle arises as an 



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