650 E. RAY LANKESTER. 



and my paper^ ' Quart. Journ. Micr. Sci./ 1890, vol. xxxi^ 

 and in no way justifies MacBride's implication that Kowalevsky 

 was right in considering the metapleural canals as coeloraic in 

 nature, and that Willey and I were wrong in denying their 

 connection with coelom. 



I will conclude this note by a quotation from the paper by 

 Willey and myself above cited (p. 455), in which the difference 

 between the actual state of things made known by us and the 

 theory of " atrial folds " so strangely resuscitated by MacBride^ 

 is set forth. 



^'It is important to point out that the mode of formation of 

 the atrium as a narrow gi'oove, which closes and sinks (as it 

 were) into the body of the Amphioxus, is really different in 

 important respects from the enclosure of a space by down- 

 growth of large folds, though ultimately, no doubt, the two 

 contrasted modes of formation come to the same thing, so far 

 as the more obvious morphological relations are concerned. 

 The mode of formation which really occurs in Amphioxus is 

 readily harmonised with the existence of the post-atrioporal 

 extension of the atrium, which gradually tapers to a fine 

 csecal canal. It also gives us an essentially different view of 

 the region called '' epipleur " by Lankester, and generally so 

 designated, from that which Bolph's theory necessitated. That 

 portion of the epipleur into which the myotomes of the body- 

 wall extend is seen now to be no downgrowth, no extension or 

 fold. It is the original unchanged body-wall, which bounds 

 the sides of the animal's body in front of the atripore, just as 

 much as it does behind. The only new growth in the atrial 

 region which takes part in the limitation of the surface is the 

 subatrial growth formed by the two little horizontal folds 

 which floor in the atrium when it is a mere canal. These in 

 the adult are represented by the region of longitudinally 

 pleated ventral wall between the two metapleura." 



