No. 3.] GROWTH AND METAMORPHOSIS OF TORNARIA. 44 1 



resemblance to Balanoglossus in all the essential points is most 

 striking, and points almost certainly to a near relationship be- 

 tween the two forms. The author points out the possible rela- 

 tionship of Cephalodiscus (and Balanoglossus) to Phoronis, and 

 is inclined to believe that Rhabdopleura shows more affinities 

 to Cephalodiscus than to the Polyzoa. 



Finally, in 1890, Lang discussed the relationship of Balano- 

 glossus to Cephalodiscus. All of the differences between the 

 two forms may be explained by the sedentary condition of 

 Cephalodiscus. The organization of Cephalodiscus shows char- 

 acteristics corresponding to the young stages of Balanoglossus. 

 But it cannot be affirmed with Harmer that Cephalodiscus is a 

 more primitive form than Balanoglossus, for it is just as easily 

 maintained that the peculiarities of Cephalodiscus would arise if 

 Balanoglossus assumed a fixed condition. There are many dif- 

 ficulties, Lang thinks, in assuming a philogenetic connection 

 between Cephalodiscus and Polyzoa (and Phoronis). 



Theoretical. 



Two conclusions we may draw, I believe, which are something 

 more than possibilities. First, that the similarities of Tornaria 

 to the Echinoderm larva, say Auricularia, are not to be 

 explained away by calling them superficial resemblances, but we 

 must conclude that they have profound morphological signifi- 

 cance. Secondly, that the relationship of Balanoglossus to the 

 Chordata is a genetic connection. The reasons for these 

 beliefs may be stated briefly, first for the relationship to the 

 Echinoderm, and secondly to the Vertebrates ; afterwards the 

 less important, because more speculative, problems may be dis- 

 cussed. Those zoologists who have worked both on Tornaria 

 and Echinoderm larvae have, as we have seen, in nearly every 

 case recognized the resemblances between the two forms. This 

 furnishes a fair argument, I think, as to the general resemblance ; 

 but if we descend into details, we find even more striking simi- 

 larities. First in importance is the identity in the two groups of 

 the anterior anteroccel and its dorsal water pore. We have 

 every reason to believe this organ to be in Tornaria a diverticu- 

 lum from the digestive tract (Bateson has shown this to be the 

 case in the larva formed by a direct development). The anterior 



