CHAPTER XXI. 



ENTEROPNEUSTA 



THE larva of Balanoglossus is known as Tornaria. The pra^larval 

 development is not known, and the youngest stage (fig. 272) so far 



described (Gotte, No. 569) has many re- 

 markable 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 

 Bipinnaria. There is a pwe-oral band, and 

 a longitudinal post-oral band ; and the two 

 bands nearly meet at the apex of the pras- 

 oral lobe (fig. 273). A contractile band 

 W. so-called water-vascular passes from the oesophagus to the apex 



/tlti f\ OTTO lrtv\i v rr oo nit /\ii^_ /> 1 i i i i t 



ol the prse-oral lobe, and a diverticulum 

 (fig. 272, W) from the alimentary tract, 

 directed towards the dorsal surface, is pre- 

 sent. 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 diverticulum 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 proe-oral lobe between 

 the two anterior bands of cilia a thickening of the epiblast (? a 

 ganglion) has become established, and on it are placed two eye-spots 

 (fig. 273 oc, and fig. 274 A). A deep bay is formed on the ventral 

 surface of the larva. 



an 



FIG. 272. EAELT STAGE IN 

 THE DEVELOPMENT OF ToENAEIA. 

 (After Gotte.) 



vesicle developing as an out- 

 growth of the mesenteron; m. 

 mouth; an. anus. 



