IX 



ENTEROPNE USTA ONTOGENY 



587 



During the further transformations which take place in the larva, the anal area 

 which is bordered by the greatly developed principal ciliated ring remains almost 

 unaltered, while the oral area, pushing out before it the preoral and the postoral 

 rings, sends symmetrical extensions (Fig. 466) into the pre- and postoral regions, 

 as follows : 



From the anterior and lateral tips of the oral region which stretch to near the 

 apical pole, two extensions, one on each side, run posteriorly and ventrally into the 

 preoral area (13), two more posteriorly and dorsally into that region of the postoral 

 area which was above distinguished as the dorsal area (2). 



In this way the larva, seen from the apical pole, has temporarily a four-rayed 

 appearance. 



From the lateral and dorsal regions of the oral area, two extensions invade a 

 postoral area dorsally (4). From the posterior lateral edges, two inconspicuous 



PIG. 466. A, B, C, Tornaria Mullen (?). A, From the ventral side ; B, from the dorsal side ; C, 

 in profile (after Spengel). 1, Apical plate, with the eyes and apical tuft ; 2, anterior dorsal lobes of 

 the oral area ; 3, " heart vesicle " ; 4, posterior lorsal lobe of the oral area ; 5, collar ccelom ; 6, trunk 

 coeloiu ; 7, anus ; 8, secondary anal ciliated ring ; 9, principal anal ciliated ring ; 10, postoral area ; 

 11, proboscis pore ; 12, proboscidal coelom (water sac) ; 13, anterior ventral lobe of the oral area, 

 14, oral area; 15, ventral "saddle"; 16, ventral zone of the postoral area; 17, anal area; 18, 

 oesophagus ; 19, stomach-intestine. 



extensions may spread posteriorly. The ventral zone, however, bulges forwards 

 ventrally towards the oral area (15). 



These changes bring about the peculiar indented course of the preoral and 

 postoral ciliated rings, shown in the figures. 



The ciliated rings may even become still more folded. Such folding reaches 

 the highest degree in Tornaria Grenacheri, hardly 1 cm. long, in which the 

 anterior ventral and the anterior dorsal extensions of the oral area bulge out at 

 the cilia-carrying edge to form numerous long, narrow, freely projecting accessory 

 lobes, resembling tentacles. 



In the frontal region, on the apical eye-bearing plate, which here becomes 

 differentiated, a tuft of delicate immobile cilia develops early. 



The larva swims in such a way that the anterior or apical pole is directed 

 upward and the anal pole downward. 



The metamorphosis of the Tornaria larva into the young Enteropneustan is 

 accompanied by the following external processes : 



