ZOOLOGY 



SECT. 



separated from one another by dorsal and ventral mesenteries, and 

 completely cut off from the proboscis-cavity. The collar-cavity 

 and also that of the proboscis are crossed by numerous strands of 

 connective-tissue of a spongy character. The collar-cavity com- 

 municates with the exterior by a pair of collar-pores ciliated tubes 

 leading into the first gill-slit or first gill-pouch. 



On the dorsal surface of the anterior part of the trunk is a double 

 row of small slits the gill-slits (Fig. 714, br.) each row situated 



brob 



dors.v 



FlG. 715. Balanoglossus. Diagrammatic sagittal section of auterior end. card. s. cardiac 

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

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

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



in a longitudinal furrow ; these slits increase in number throughout 

 life. The most anterior are in some species overlapped by a 

 posterior prolongation of the collar called the operculum. A 

 pair of longitudinal genital ridges (gen.) not recognisable in some 

 species which extend throughout a considerable part of the 

 length of the body both behind and in the region of the gill-slits 

 (branchial region), are formed by the internally situated gonads : 

 these ridges are so prominent in some of the genera as to form a 

 pair of wide wing-like lateral folds. Behind the branchial region 



