METAMORPHOSIS OF ASTERINA GIBBOSA. 233 



For the sake of continuity we shall follow the changes 

 which the secondary left posterior enterocœl undergoes one stage 

 further, although some important changes are taking place else- 

 where. In stage E/F (fig. 13) the passage of communication on 

 the right side just referred to is cut off" by the formation of a 

 new mesentery (fig. 13 d & e), and the passage on the dorsum 

 has also been much constricted, as may be seen in fig. 13 (/. 

 The separation is effected immediately afterwards, it taking place 

 along a line drawn between a valve-like process next the pore- 

 canal and the posterior extremity of the hydrocœl lobe 5 in the 

 figure just mentioned. AVhen this separation has been effected 

 we have three entirely distinct cavities, the anterior, the epigas- 

 tric, and the secondary left posterior, enterocœl. 



We must now^ turn back to stage C/D and direct our at- 

 tention to what has been called the "dorsal sac." Accordinp- 

 to MacBride ['96, p. 349] this vesicle is budded off from the 

 anterior cœlom at its posterior extremity, and for this reason, 

 as well as from some observations on aljnormal larvae, he regards 

 it as iho right hydrocœl. In Asterlas pallida 1 stated that 

 this sac was budded otf from the left enterocœl directly behind 

 the pore-canal,''' and that it was subsequently observed to be an 

 extension of the anterior enterocœl ['98 a, p. 254]. In Asierina 

 gibbosa I also find the sac originate in exactly the same way, 

 but it remains independent all through its subsequent changes. 

 In fig. 17 I have reproduced four successive sections passing 

 through the dorsal sac, when it is being formed. Although the 

 wall of the dorsal sac is closely appressed to that of the right 



'■'"There I added the explauatoiy words, "that is to say at the boundary hetweeu tlir 

 posterior and anterior enteroccels." This was called t'ortli by tlie subsc(|uent displacement 

 of the sac referred to in tlie text, in ciinse<|nenc(' dl' wbirli it came tn o|)en intu llic anterior 

 euteroeœl. 



