1 24 RITTER 



gut which Morgan shows in fig. 64, of a section taken from the 

 extreme posterior portion of the collar region of his oldest young 

 Balanoglossiis. The author mentions that the histological dif- 

 ferentiation peculiar to the anterior pouch of the notochord, ex- 

 tends to the ' strongly marked dorsal groove ' of the esophagus. 

 There is no room for doubt, then, that both Bateson and Mor- 

 gan have seen the esophageal notochord in the young Balano- 

 glossus of both the species studied by them. And since these 

 two investigators have carried out their studies more completely, 

 both as to methods and details, and also as to stages of advance- 

 ment in the ontogeny of the animal, than have any other zoolo- 

 gists, we may confidently predict that fuller knowledge of the 

 life history of other species will reveal its presence generally ^ 

 at so7ne stage of development^ in the entire group of Enter- 

 opncusta. 



REMARKS ON THE PRIMITIVENESS OF THE BALANOGLOSSID^. 



While, as already said, it is not my purpose to discuss the 

 theoretical bearings of the facts presented in this paper, one 

 point is so close at hand that I cannot ignore it entirely. The 

 fact that the esophageal notochord is present, highly developed, 

 in the adult of Harriniania^ while it exists only in the embryos 

 of other species, and disappears wholly or almost so in the adult, 

 is in itself strong evidence of the primitiveness of the species, 

 and I conclude from this and other reasons that Harrimania 

 stands at the very bottom among living species of the Enterop- 

 neiista. But this view is in direct opposition to Willej^'s, who 

 sees in the Ptychoderida; the most primitive forms, and in the 

 Balanoglossidai the most modified, and hence the oldest, forms. 

 After carefully considering the evidence on which Willey bases 

 his views, the facts relating to the notochord left entirely aside, 

 I must believe that his conclusions will not stand. That the 

 species of animals of a homogeneous and well circumscribed 

 group like the Enteropneusta, which are clearly the most highly 

 differentiated and specialized, and are on the whole by far the 

 largest in size, should be regarded as the most primitive ; while 

 the simpler, smaller species are held to be the oldest and most 

 modified, and this in face of the fact that all have essentially 



