THE PHYLA HEMICHORDATA AND CHORDATA 



Fig. 18.5. Compound ascidians. A, 

 colonies of Bolryllus schlosseri; the white 

 stripes radiate from the common, central 

 excurrent opening to the incurrent open- 

 ing of each individual. B, diagrammatic 

 vertical section through a colony, show- 

 ing the relation of the individuals to the 

 entire group. (.4, photograph by George 

 Lower; B, after O. Seeliger, from W. 

 Stempell, 1926, ^oologie im Gnmdnss.) 



Pharynx 



Stomach 



Epicardium 



- Testis 

 Ovary 



The notochord is resorbed along with the tail; the nerve cord coalesces to 

 form the ganglion of the adult. The gill slits of the larva are subdivided 

 and modified to form the many openings of the adult pharynx. This mode of 

 development is most reasonably explained by the assumption that the modern, 

 sessile tunicates have descended from primitive chordate ancestors that were 

 not attached, but free-swimming, like the tadpole. It is on this basis, and 

 because of the presence of such unequivocal chordate characteristics in the 

 larva, that tunicates are classified within the phylum Chordata. 



In contrast with the solitary ascidians, such as Moloida, Slycla, and Ciona, 

 the so-called compound ascidians are forms in which budding produces 



549 



