254 



THE PROTOCHORDATA. 



\~l.c.l. 



oe-j 



BROOKS and G. W. FIELD in the larvae of a common star- 

 fish, Asterias vulgaris. In this case the primary enteroccel 

 becomes constricted off from the archenteron in the form 

 of two equal pouches. The right and left enteroccelic sacs 

 then take up a symmetrical position on each side of the 

 larval oesophagus, and each sac next opens to the exterior 

 by a water-pore. The pore in connexion with the right 

 sac (Fig. 12 1 ) is, however, of a transitory, rudimentary 



character, and soon closes 

 up, while the left pore per- 

 sists as the definitive water- 

 pore. As in Tornaria, so 

 here, the cavity of the larval 

 body generally, and of the 

 praeoral region (prceoral lobe) 

 in particular, is the primary 

 body-cavity or blastoccel, 

 and contains scattered mes- 

 enchyme-cells. At a later 

 Fig. 121. - Young larva of Asterias stage in the larva of As- 



vulgaris, from the dorsal side. (After ter i as tne right and left 

 G. W. FIELD.) 



/./. Praeoral lobe, l.c.b. Circumoral enterOCCelic SaCS, having ill- 

 (longitudinal) ciliated band. oes. CEsoph- , , . , . 



agus! r. e . and /.,. Right and left en- creased greatly in length, 



terocoslic sacs, each opening by a " water- meet one another in the 



pore " to the exterior, sf. Stomach, inf. 



Aperture, leading from stomach into in- region of the praeoral lobe 



and fuse together, thus put- 

 ting their two cavities into communication across the 

 median line. The median portion of the enteroccel thus 

 produced extends up into the praeoral lobe, and so the 

 primary blastoccelic cavity of the latter is replaced by a 

 secondary ingrowth of the enteroccel (Fig. 122). 



Similarly with the metamorphosis of Tornaria, the 

 anterior enterocoel, which is at first of very inconsid- 



