EAELY STAGES IN DEVELOPMENT OF BALANOGLOSSUS. 231 



string is, however, of much more importance, and a full dis- 

 cussion of the meaning of it must be deferred. But in com- 

 paring the modified anterior mesoblastic sac in the opaque 

 larva with the water vessel of the larvse of Asteroids, &c,, one 

 point is obviously remarkable and may conceivably be of some 

 morphological value, viz. that while in Asteroid larvse the 

 water vessel is developed always from the left primitive archen- 

 teric diverticulum so in Tornaria the original evagination 

 from the gut to the exterior is on the left side of the body 

 (Gotte). In the larva and adult of Balanoglossus (sp. incert.), 

 and in the adult of B. minutus the permanent opening com- 

 municates with the left posterior horn of the anterior body 

 cavity the cells of which become columnar, while those of the 

 right horn have the structure of ordinary connective tissue. 

 The origin of the water vessels from the left vesicle is also true 

 generally speaking for Echinoidea and Ophiuroidea. It would 

 therefore appear to be of some morphological importance. 

 Upon these and other points in the relationship of Balano- 

 glossus to other forms, the later development may be expected 

 to afford some information. 



There is one more comparison which may, I think, be shortly 

 alluded to since it is suggested by even a superficial examina- 

 tion of the early stages of this opaque larva. 



Since Balanoglossus possesses gill slits which are not 

 comparable with any structures present in animals outside the 

 Chordata, it appears prima facie as worthy of consideration 

 whether the presence of these structures may not point to a 

 common origin. 



Now leaving this question aside for the present, I would 

 suggest that a very striking similarity does exist between the 

 general history of the early development of this larva and that 

 described by Hatschek for Amphioxus, this resemblance being 

 more particularly strong in the situation and mode of origin 

 of (1) the central nervous system and of (2) the mesoblastic 

 somites. 



For according to Hatschek' s account of its origin, the nervous 

 system in Amphioxus differs from the central nervous system in 



