254 s. GOTO; 



radial water-tubes, and their walls are considerably thickened. I num- 

 ber them as marked in the figures, i. e., I begin on the ventral side 

 and proceed counter-clockwise^^ as seen from the left side. No. 2 is 

 the first developed and is situated hindmost, 1 and o are formed almost 

 simultaneously, then 4, and lastly 5. The latter is situated anteriorly 

 to the opening of the pore-canal into the enterocœl and is somewhat 

 removed from the dorsal margin of tlie left enterocœl. 



In Bipinnaria asterigem Bury ['95, p. 66^ has observed that 

 the ventral horn of the left posterior cavity curves over to the right 

 side and fuses with the right body-cavity ; and he adds, " This fusion 

 makes it impossible to determine the exact limit of the two cavities 

 (right and left), but we shall probably not fall into any important 

 error if we regard the point of junction as median." In Ästerias 

 pallida this fusion is eifected in the next stage and the line of fusion is, 

 as can already be inferred from the foregoing description, wholly on 

 the right side of the body. 



It remains to speak of the " dorsal sac"-^ observed by previous 

 students in this stage [Field '93, Bury '87 & '95, MacBride '96]. 

 Field and Bur}^ agree in regarding it as of mesench^nnatous origin ; 

 MacBride, on the other hand, hns advanced an entirely different 

 opinion. According to him it represents the right hydrocô?! and is 

 budded off from the right posterior extremity of the anterior cœlom, 

 with which it is, for some time, connected by a string of cells ; this 

 string of cells, however, soon atrophies and the cavity in cjuestion is 

 then left as a closed vesicle in the midst of the mesenchyme ['96, p. 

 349]. Further in MacBride's opinion " the right hydrocœl persists 



1). This is the same method of nuuiberingas that used by Ludwig. MacBride, on the other 

 hand, begins at the other end and proceeds clockwise. 



2). Called by Bury in his paper of '87 " blood vascular space." " Dorsal sac " is the 

 name employed by him in his later paper ['95]. 



