74 



INTRODUCTION TO EVOLUTION 



opening of atrium 



anus 



mouth 

 pharyngeal gill silts 



tunic 



heart 



FIG. 4.16. Internal anatomy of an adult tunicate (Molgula). Diagrammatic. 

 (From Buchsbaum, Animals Without Backbones, 2nd ed., University of Chi- 

 cago Press, 1948, p. 316.) 



slits," is ejected into the sea again through the excurrent siphon. This 

 method of feeding is much like that of such molluscs as clams and oysters. 

 Accordingly tunicates were at one time considered molluscs or close 

 relatives of molluscs. 



Later the young or larval stages of tunicates were discovered. Unlike the 

 adult, the larva of Molgula (Fig. 4.17) is a free-swimming, tadpolelike 

 creature. Its swimming tail is stiffened by a notochord; it has a pharynx 

 pierced with gill slits; and it has a small brain and spinal cord dorsal in 

 position. These are the most distinctive characteristics of Phylum Chordata 

 (the phylum to which all vertebrates belong) and are most unlike struc- 

 tures possessed by members of Phylum Mollusca. Evidently, therefore, 

 tunicates belong in Phylum Chordata. The larval condition appears to in- 

 dicate that their ancestors were free-swimming chordates. The present 

 mode of life of adult members of Molgula, unmoving, with a mollusclike 

 method of feeding, must represent a specialized condition adopted by the 



