358 R. W. MACBRIDE. 



The rudimentary character of the ventral and lateral walls 

 of the pharynx of the larva accounts for the apparently 

 venti-al origin of the atrial wall. This wall always arises at 

 the spot where the flattened epithelium passes into the high 

 ciliated branchial epithelium. 



Since the view of Lankester and Willey has been shown to 

 be untenable as far as the phai^yngeal region is concerned, 

 it is very unlikely that it is true for the posterior region of 

 the animal. If we examine a just metamorphosed individual 

 we find that near the atrial pore the atrial cavity is com- 

 pletely ventral to the intestine, whilst further forward the 

 cavity half surrounds it. If, however, we measure the dis- 

 tance from the mid-dorsal line of the gut to the dorsal 

 edge of atrial cavity in the two regions, we shall find it about 

 the same, so that the apparent upgrowth of the atrial cavity 

 is largely if not entirely accounted for by the increased size 

 and downward extension of the ventral part of the gut.^ 



Lankestei' and Willey state further that their account of 

 the growth of the atrial cavity ''is readily harmonised with 

 the existence of the post-atrioporal extension of the atrium 

 which gradually tapers to a fine c£ecal canal." I must say 

 that I hold on this subject an absolutely contrary view. 



In an individual which had just completed the metamor- 

 phosis there is no trace of this post-atrioporal extension. 

 In the region of the atriopore we find in such a specimen 

 four ventral ridges. The two outer are themetapleural ; the 

 two inner, on the contrary, are the separated walls of the 

 atrium, here hanging vertically down. In an individual, on 

 the other hand, in which the genital organs are commencing 

 to appear, the post-atrioporal connection can be traced back 



• Lankester and Willey ^Ive figures in the text of their paper illustrating 

 the manner in which, according to them, the upgrowth of the atrial cavity 

 divides the splanchnocoele into the inner or splanchnic and an outer or pleural 

 portion. I can find in my sections no traces whatever of this outer pleural 

 portion. Can Lankester and Willey refer to the genital sac ? This has of 

 course been shown to be formed not from the splanchnocoele at all, but from 

 the myotome. 



