r 
198 Balanogtlossus. | ZOE 
break through to communicate with the outside world as in the 
adult animal, till a later period, after the metamorphosis has begun. 
While these new organs have been developing the old ones have 
been increasing in size and form. The water vesicle has elongated 
lengthwise of the animal; its walls have thickened in some regions, 
and as seen in Fig. 4, at X, a pair of horn-like processes now ex- 
tend downward and a little backward, straddling the cesophagus. I 
would call particular attention to these because they have been seen 
and figured by Fewkes,” but their existence has been denied by 
Morgan.” 
_ The changes that take place during the metamorphosis can here 
be touched upon only in the briefest way. The Tornaria loses its 
transparency, largely; gives up its free swimming career and settles 
down to the bottom of the vessel in which it is contained; its cilia 
disappear, and with them the thickened bands on which they are 
situated; the whole larva elongates, the anterior portion to become 
the proboscis, and the region behind the circular band of cilia to be- 
come the abdomen. The gills, which in the Tornaria are far for- 
ward, are brought to the position in which they are found in the 
adult, viz.: behind the collar, by the drawing backward of the 
stomach and cesophagus during the transformation. Figure 3 
represents a young Balanoglossus about as far advanced as has yet 
been obtained by keeping them in confinement. The transforma- 
tion to this stage takes place quite rapidly when once it sets in, but 
beyond this it seems to proceed very slowly. In fact, in the artificial 
conditions of the aquarium the little animal seems determined not 
to develop much further. 
As already said, in the species the development of which was 
studied by Mr. Bateson, there is no Tornaria stage. It is in this 
species only that the method of cleavage and formation of the 
blastula and gastrula are known. In these early stages the pro- 
cesses, are very similar to those which take place in Amphioxus and 
the Tunicates. 
The very interesting question at once arises—it being remembered . 
that the adults of all species are so nearly alike as to have never 
J. W. Fewkes. On the Development of Certain Worm Larve. Bull. Mus. 
2 Comp. Zool., Harvard University, Vol. xi, 1883. 
16]. c. 
