84 WILLIAM BATESON. 



like one another, but apparently very unlike any other group 

 of animals. Before Tornaria was known to be a stage in their 

 development they were assumed to be worms of some kind, but 

 after Metschnikoff had succeeded in proving Tornaria to be 

 the larva of a Balanoglossus this was felt as an impossible view 

 of its affinities. Up to this time Tornaria had been regarded 

 by Job. Miiller, who first described it ('Berl. Akad.,' 1849, 

 1850), and by others who examined it as a varied form of 

 Bipinnaria, which, indeed, it very closely resembles, differing 

 only in the presence of eye-spots, and of a peri-anal ring of 

 cilia j both of which structures are liable to great variation. 

 When, then, Metschnikoff" discovered its real destiny, it 

 appeared at first sight necessary to suppose the Enteropneusta 

 closely connected with the Echinodermata, and accordingly 

 Metschnikoff (' Zool. Anz.,' 1880) proposed to include them 

 in a division Bilateralia under the Echinodermata, the re- 

 mainder of the group forming a parallel division, Radiata. 

 But this generalisation with regard to the group was made 

 solely on the characters of the larva, and almost without 

 reference to the structure of the adult, which, indeed, was 

 little known. So certain, however, did the conclusion seem, 

 that Metschnikoff was led to suppose that the gill-slits of 

 Balanoglossus were mere amplifications of the water-vascular 

 system of Echinoderms, which could hardly have been sug- 

 gested had it not been felt that no other solution was possible. 

 Since this time the anatomy of the adult has become more fully 

 known, and another mode of development has been shown to 

 occur, and from neither of these additional sets of facts can 

 any confirmation of the Echiuoderm theory be derived. Hence 

 we must conclude that the characters of Tornaria are not to 

 be looked to solely in attempting a solution of the problem. 



In the development of Balanoglossus Kowalevskii the 

 following important features occur : (1) the origin of the cen- 

 tral nervous system is by longitudinal delamination from the 

 skin in the dorsal middle line ; (2) at the anterior end of the 

 body a portion of hypoblast is constricted off on the dorsal 

 side to form a supporting structure, i. e. a notochord ; (3) the 



