BALANOGLOSSUS. 



301 



the ectoderm, and sinks into a depression of the water vascular 

 vesicle. At the apical pole there is a thickening of the ectoderm 

 resembling the apical plate of the larval Worms and containing two 

 eye-spots. 



The development of the larva into the adult Balanoglossus was 

 first traced by E. Metschnikoff and then by A. Agassiz. The band of 

 cilia gradually disappears, the prse-oral part of the larva becomes the 

 proboscis, while the oral portion gives rise to the collar. The trunk 

 is formed from the posterior elongated portion on which the posterior 



transverse ciliated band still persists. The 



anterior portion of the alimentary canal 



becomes pierced by paired branchial slits 



(figs. 243, 244). 

 The body of 



the adult animal 



is worm-like and 



completely cili- 

 ated ; it can be 



divided by the 



external features 



into a number of 



different regions. 



The anterior end 



of the body is 



indicated by a 



proboscis well 



marked off and 



projecting like a 



This is fol- 



Fio. 244. Stage in the conver- 



sion of Tornaria into Balano- 

 ylossui, with four pairs of 

 branchial slits (after Al. 

 Agassiz). Letters as in figs. 

 242, 243. 



FIG. 243. Stage in the con- nea( l- 



version of Tornaria into Bala- lowed bv a niUSCU- 

 noylusiiit, with one pair of 



branchial slits (after E. Met- iar Collar. Jb*Oste- 

 schuikoff), seen from the side. r i or to t ] ie co llar 

 So, external branchial open- 

 ing ; p, peritoneal sac ; re, there is a longer portion of the body, the 

 circular vessel; o, mouth; c, branchial region, which may be divided into 



he\rt. 



a median distinctly ringed part (branchiae) 



and two lobed lateral portions usually filled with yellow glands. At 

 the boundary, between the median portion and the two lateral lobes, 

 there are on either side rows of openings which serve for the exit 

 of the water from the branchial chamber. Then follows a third 

 division of the body, the gastric region, upon the upper side of 

 which there are four rows of yellow glands (generative glands). 



