10J5 



Such a primitive organ as the mesial buccal opening of the Verte- 

 brates must howe\e!- originally have been present in Amphioxns. 

 In my opinion the opening of Hatschkk's groove, which is a product 

 sepai-ated off from the fore-end of the intestine, must be considered 

 as the primitive month of Amphioxns. 



However this may be, the original month of Amphioxns nnist 

 have been, just as in the higher animals, a symmetrii'al organ 

 situated in tiie mesial plane, rostrally to the spiracle. 



Why has the original month in Amphioxns been lost and been 

 replaced by the left spiracle? 



The manner in which the larvae that have jiisl left the egg 

 membi'anes move, can show us the way to come to an answer of 

 this question. These larvae move forward spirally, by means of 

 cilia, turning round the longitudinal axis from i-ight to left. If 

 we siip|)Ose that ancestors of Amphioxns possessed this form of 

 motion permanently, then the left spiracle occun-ed in a more 

 advantageous position to take in the sea-water necessary for nutrition 

 and respiration, than did the mesially situated primitive buccal 

 opening. This foi'm of motion also makes the i-emarkable occtirreiu'e 

 of the branchial apertures comprehensible. If we neglect tiie fore- 

 most pair, which is metamorphosed to mouth and club-shaped gland, 

 the gill clefts do not occur in successive pairs as in all the Craniota. 

 On the contrary, the a|)erttires on the right side are altogether wanting 

 in the larva during the period of growth; they do not appear until 

 during the metamorphosis. During the period of larval growth the 

 clefts on the left side alone appear. They make their appearance 

 successively to a total of 14 or 15, apparently however not on the 

 left side but in the topoifrapliical mesial plane of the pharynx. 

 Shortly after appearing each of these clefts is removed to the topo- 

 graphical right side. 



At first sight this phenomenon seems very extraordinary, but it 

 becomes more comprehensible when we considei- the location of the 

 truncus arteriosus, which indicates in Amphioxns, as in the higher 

 animals, the morphological mesial plaiije of the pharynx. 



In the older larva of Amphioxus the truncus arteriosus does not 

 run in the plane of symmetry under along the branchial intestine, 

 but high dorsally along its right side. 



The morphological right side has remained behind in development 

 as a narrow dorsal strip in consequence of the excessive growth in 

 breadth of the left side, which has hereby occupied the ventral 

 territory of the right side. 



Although the first 14 or 15 bi-anchial apertni-es of the larva now 



6G 



Proceedings Royal Acad. Amsterdam, Vol. XXI. 



