22(i ABTHUE WILLEY. 



Structures of first importance are relegated to foot-notes, e. g. 

 the genital pleurae (p. 5) and roots (p. 45) of Ptychoderidse. 



In a paper published in this Journal,^ which our authors 

 have overlooked, I have given reasons for supposing that a 

 form like Ptychodera flava, in which the gill-slits open 

 freely to the exterior and not first into gill-pouches, represents 

 the most primitive existing type of Enteropneusta. Speaking 

 from personal experience, I may at least say that this species 

 has opened my eyes as to the significance of the enteropneustic 

 organisation. 



The figure of Ptychodera clavigera, given on pi. ii, 

 facing j)age 64, is apt to be misleading. The pharynx in this 

 figure appears to stand boldly forth as a cylindrical tube at the 

 base of the open chamber formed by the arching genital 

 pleurae; and the parallel arcuate lines have the appearance 

 whicli is actually presented by the true gill-bars in Ptyclio- 

 dera flava. The gill-bars of Pt. clavigera are, however, 

 quite invisible externally, the genital pleurae have a dorsal 

 origin, the pharynx does not project beyond the level of the 

 floor of the peribranchial space,^ and the gill-pores are 

 extremely small, lying at the base of the narrow branchial 

 grooves. 



The authors have very naturally followed Speiigel in their 

 explanation of the lateral septa of the Ptychoderidse, as being 

 the outer walls of a pair of coelomic diverticula (p. 23, foot- 

 note). 



The i)rolongations of the truncal coelom into the collar 

 region (viz. periheemal and peripharyngeal cavities) are intelli- 

 gible facts ; but how the truncal coelom could project a portion 

 of itself into itself was a mystery to my mind until I realised 

 that there is no question of a diverticulum at all. 



1 A. Willey, on Ptychodera flava, Esclischoltz, 'Quart. Journ. Micr. 

 Sci.,' vol. 40, 1897, p. 165. 



' Excepting that tlie branchial tract or gill-area, i. e. the area enclosed 

 within tlie branchial grooves is somewhat arched. The blue-lined stiiict.iire 

 in the figure above referred to is simply the gill-area or Kieiiienfcld of 

 Speugel. 



