552 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 Joh. 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; 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 Echinoderm 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 
