CHAPTER XXI. 



ENTEROPNEUSTA. 



THE larva of Balanoglossus is known as Tornaria. The prae- 

 larval development is not known, and the youngest stage (fig. 

 272) so far described (Gotte, No. 569) has 

 many remarkable points of resemblance to 

 a young Bipinnaria. 



A mouth (m), situated on the ventral 

 surface, leads into an alimentary canal with 

 a terminal anus (an}. A prae-oral lobe is 

 well developed, as in Bipinnaria, but there 

 is no post-anal lobe. The bands of cilia 

 have the same' general form as in Bipin- 

 naria. There is a prae-oral band, and a 

 longitudinal post-oral band ; and the two 

 bands nearly meet at the apex of the prae- 

 oral lobe (fig. 273). A contractile band 

 passes from the oesophagus to the apex of 

 the prae-oral lobe, and a diverticulum (fig. 272, W) from the 

 alimentary tract, directed towards the dorsal surface, is present. 

 Contractile cells are scattered in the space between the body 

 wall and the gut. 



In the following stage (fig. 274 A) a conspicuous transverse 

 post-oral band of a single row of long cilia is formed, and the 

 original bands become more sinuous. The alimentary diverti- 

 culum of the last stage becomes an independent vesicle opening 

 by a pore on the dorsal surface (fig. 274 A, w). The contractile 

 cord is now inserted on this vesicle. Where this cord joins the 

 apex of the prae-oral lobe between the two anterior bands of 

 cilia a thickening of the epiblast (? a ganglion) has become 



372 



an 



FIG. 272. EARLY 

 STAGE IN THE DEVELOP- 

 MENT OF TORNARIA. 

 (After Gotte.) 



W. so-called water- 

 vascular vesicle develop- 

 ing as an outgrowth 

 of the mesenteron ; ;;/. 

 mouth; an. anus. 



