256 s. GOTO; 



it and the right enterocœl having disappeared altogether (Fig. 7, PI. 

 XIX, Fig. 17 a & /), PI. XX). There is, however, an incomplete valve- 

 like septum at the posterior end of this common cavity in about the 

 same position as the boundary of the right })osterior enterocœl in stage 

 B. Again, looking at the larva fmm the right side (Fig. 8, PI. XIX), 

 we see that a portion of the right posterior enterocœl has Ijeen cut off IVom 

 the rest at the dorsal posterior corner of the body, fcjrming here a 

 crescent-shaped closed cavity extending from a little in frontofthe water- 

 pore to the posterior end of the body. From the position which it oc- 

 cupies in the complete star I shall call it " epigastric enteroc(el." The 

 mesentery lying between it and the rest of the body cavity on the right 

 side has no genetic connection wliatever with the primary mesentei'v 

 separating the right from the left posterior enterociel, as will be clear 

 from the following description explaining its formation. 



The very beginning of the septum in question can already be 

 observed in stage H. Fig. 21, PI. XXI, represent-^ a portion of a trans- 

 verse section of a specimen in such a stage, some seven sections (each= 

 6-7 /■«) behind the water-[)ore. The left posterior enterocœl extends 

 here over the mid-dorsal line — a i-esult of an active proliferation of the 

 cells of its dextro-dorsal wall, with which the formation of the " dorsal 

 sac" is connected. [n a somewhat dorsal portion of the right entero- 

 cœl one sees a somewhat indistinct septum formed by the coalescence 

 of the epithelium. It rmis through only a few sections, and at [)laces 

 there is no septum, but tlie walls of the two sides are simply closely 

 ap]3ressed to each other. The latter condition persists to a rather late 

 stage in the posterior portion of the body, although in the more 

 anterior part, /.{'., near the water- pore, the mesentery (using the teian 

 with its usual connotation, /.e., two layers of epithelium with mesen- 

 chyme between) is completed. Compare Figs. 2o&24, PI. XXI, the latter 

 of which is three sections behind the level at which, in this particular 



