198 Balanoglo&sus. ( 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 oesophagus. 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 oesophagus 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 Larvx. Bull. Mus. 



Comp. Zoo!., Harvard University, Vol. xi, 1883. 

 16 1. c. 



