84 VERTEBRATE ZOOLOGY 



ancestry. The adult Balanoglossus, and even more so some of its 

 relatives, such as Harrimania, shows certain striking chordate re- 

 semblances. The branchial orifices are similar in form and relations 

 and remind one of the pharyngeal clefts of Amphioxus; the notochord 

 is somewhat doubtful in Balanoglossus, but in Harrimania it appears in 

 a much more obvious form and has an origin quite like that in Amphi- 

 oxus; the nervous system is highly generalized, consisting of four 

 longitudinal cords, with the dorsal somewhat more strongly developed 

 than the rest. In Glossobalamis and Ptychordera, according to 

 Harmer, "a central canal, opening in front and behind, exists through- 

 out the entire length of the central nervous system. . . . Balanoglos- 

 sus is thus typically provided with a dorsal, tubular, central nervous 

 system, and although it does not extend beyond the limits of the 

 collar, it shows noteworthy resemblances to Vertebrate Animals." 



It is the opinion of many students of comparative anatomy that 

 Balanoglossus is a chordate or pro-vertebrate more primitive than 

 Amphioxus. According to Wilder, Balanoglossus and its relatives 

 the Enteropneusta "lie nearly in the line of vertebrate descent, and 

 represent an earlier stage than that of the tunicates. But here the 

 chain seems to end, for Balanoglossus is itself unusually isolated and 

 shows no close affinity to any other invertebrate types." 



Although we may accept as unquestioned the above view of the 

 chordate affinities of Balanoglossus there are also very striking resem- 

 blances between the latter and certain echinoderms which seem to 

 the writer to be as weighty as are those relating it to the chordates. 

 Foremost of these echinoderm resemblances of Balanoglossus is that 

 involved in the structure of the larvae of the two groups. The Torn- 

 aria larva of Balanoglossus (Fig. 46) is compared with the Aurimlaria 

 larva of the holothurian and the Bipinnaria larva of the starfish. The 

 resemblance is obvious. Moreover, there is in Balanoglossus a system 

 quite like the water vascular system of the echinoderms, which is 

 unique for that group. It will be recalled that there is a water-pore 

 communicating with the proboscis coelom and a pair of water-pores 

 communicating with the paired collar coeloms. "Recent studies on 

 the development of Echinoderms," says Wilder, ' have made it prob- 

 able that the five body cavities of Balanoglossus are represented in the 

 larvaj of these animals; and this materially strengthens the probabil- 

 ity of the view that the respective adults are also allied. It may be 

 added that the relationship which appears to be indicated is between 



