TYPE PROTOGHORDATA. 



605 



tongue bars belonging to each arch are connected by trans- 

 Terse bars (synapticula, s) which extend across the valves of 

 the branchial slits. 



The digestive tract is practically a straight tube extending 

 from the mouth, situated on the ventral surface of the neck 

 of the proboscis, to the terminal anus. From the dorsal wall 



of the oesophagus a finger- 

 shaped diverticulum arises, the 

 notochord, which extends for- 

 wards into the proboscis; its 

 lumen, in its terminal portion, 

 being practically obliterated by 

 the vacuolization and enlarge- 

 ment of the cells which line it 

 and which are continuous with 

 the endodermal lining of the 

 digestive tract. In the anterior 

 portion of the trunk region the 

 branchial slits already referred 

 to occur, arising as diverticula 

 from the dorso-lateral portions 

 of the intestine (Fig. 275, JB, 

 kh) and eventually opening to 

 the exterior {kp). More poste- 

 riorly in some species the wall of the intestine is pouched out 

 into sacculations which have been regarded as liver-sacs. In 

 Balanoglossus (Glandiceps) hacksi an accessory intestine occurs, 

 arising in the middle of the liver region from the dorsal sur- 

 face of the intestine in the form of a tube, which, more poste- 

 riorly, opens again into the intestine. It recalls the accessory 

 intestine of certain Annelids, Gephyreans and Echinoderms, 

 but is peculiar in being dorsal in position. In certain species 

 also paired or unpaired communications of the intestine with 

 the exterior have been found, usually arising in the liver 

 region, and opening upon the dorsal surface ; their signifi- 

 cance is at present unknown. 



A well-developed nervous system is present in the form of 

 an elongated cord lying in the mid-dorsal line of the collar 

 region, with the ectoderm of which it is in connection at 



Fig. 276.— Diagram of the Bran- 

 chial Skeleton of Balanoglossus 

 (after Spengel). 



« = syuapticula. 

 sb = septal bar. 

 ib = tongue-bar. 



