ZOOLOGY 



SECT. 



supply the proboscis. In communication with the sinus are a 

 number of vessels of a bilateral plexus the glomerulus situated at 

 the anterior end of the alimentary diverticulum. From the poste- 

 rior end of each half of the glomerulus there passes backwards 

 an efferent vessel which breaks up into a plexus ; the two plexuses 

 unite ventrally to form a median ventral plexus continuous behind 

 with the ventral vessel. The dorsal sinus, having no definite walls 



.ft rob 



div 



dcrs.v 



FIG. 665. Balanoglossus Diagrammatic sagittal action of anterior end. card. s. cardiac 

 sac ; div. diverticulum (supposed notochord) ; dors. n. dorsal nerve strand ; dors. sin. dorsal 

 sinus ; dors. v. dorsal vessel ; mo. mouth ; prob. proboscis ; prob.po. proboscis pore ; prob. skel. 

 proboscis skeleton ; vent. n. ventral nerve strand ; vent. v. ventral vessel. (After Spengel.) 



is not contractile ; but a closed sac, the cardiac sac (card, s.), 

 derived from the heart of the larva and situated on the dorsal side 

 of the sinus, has a muscular ventral wall by the contractions of 

 which the blood may be propelled. 



The nervous system consists of dorsal and ventral strands 

 (dors. n.,vent. n.) which extend throughout the length of the body. 

 These are merely thickenings of a layer of nerve-fibres which 

 extends over the entire body below the epidermis the thickening 

 being enclosed on both sides by a layer of cells which passes into 



