318 ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



As is well known it is Gegenbaur's view that the method of delamination is more 

 primitive than the method of fusion of medullary folds. There are others who hold 

 the opposite view. By considering how far the one or the other view will lead him 

 to an appreciation of the subject in hand, the reader may choose for himself between 

 the two views. If he chooses the method of delamination, then he takes upon him- 

 self the onus of explaining the meaning of the central canal. For my part I pin 

 my faith to the medullary folds because by their means I see my way to the appli- 

 cation of the principle of change of function, and to an approximate conception of 

 the meaning of the central canal. 



That the genital pleurae are structures which are capable of undergoing change 

 of function is shown by various indications; perhaps more than anything by the fact 

 that they already serve at least two functions, namely, the supreme function of carry- 

 ing the gonads and the secondary function of protecting the branchial complex. That 

 they do serve the latter function is quite obvious in a form like Ft. flava, while in 

 a form like Pt. carnosa, where it is less obvious, it is none the less indicated by their 

 capacity of uniting together over the gill-area by a mucous junction (PI. XXVII. Fig. 6). 

 The capacity for change of function is also strikingly exhibited at the anterior 

 end of the genital pleurae in species of the subgenus Tauroglossus, where they con- 

 verge towards one another dorsally in the region of the posterior neuropore and no 

 longer contain gonads in this region (cf. PI. XXXII. Fig. 61). 



Spengel has drawn attention to the more ventral position of the gill-slits in 

 Amphioxus as compared with their more dorsal position in the Enteropneusta, and 

 naturally uses this as an argument in favour of his views. That there is a difference 

 I gladly admit. A process of readjustment has been at work 1 . The dorsal gill-pores 

 of the Enteropneusta are not present in Amphioxus. 



It is a truism to say that change of function of an organ is and must be ac- 

 companied by correlated changes of organisation. 



To take the particular case under discussion as an example it may be said that 

 the change of function by which the genital pleurae could become converted into 

 medullar}- folds would be accompanied by their complete emancipation from the gonads 

 and, sooner or later, by the abolition of the dorsal gill-pores 2 , the gill-clefts finding 

 another (ventral) outlet. 



Analogous changes have apparently actually taken place in the collar; this is 

 seen in cases of regeneration and may also be inferred on other grounds (see below 

 p. 321). 



It now becomes necessary to discuss the organisation of Amphioxus in the light 

 of the above considerations. 



We have seen that the pleural folds of the Ptychoderidae possess gonadial, 

 medullary and peribranchial qualities. Taken as a whole therefore they constitute, 

 potentially, a complex primordium. We have already dealt with their gonadial and 

 medullary attributes and it only remains to consider their peribranchial potentialities. 



1 Compare the excessive readjustment of the gill-clefts which takes place in the ontogeny of Amphiojcus. 

 - There are two ways of abolishing inconvenient gill-openings, namely, (1) by closure of the slits, (2) by 

 readjustment of the slits. Both these methods are adopted in the larva of Amphioxus. 



