ZOOLOGY 



SECT. 



of a spongy character. The collar-cavity communicates 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. 705, br.) each row situated 

 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. 



div 



FIG. 706. BalanoglOBSUS. 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; f\ ob. proboscis ; prob. po. proboscis-pore ; prob. skel. 

 proboscis-skeleton ; vent. n. ventral ner'se strand ; vent. v. ventral vessel. (After Spengel.) 



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 

 are two rows of prominences (hep.) formed by the hepatic caeca. 

 The trunk is irregularly ringed, this annulation, which is entirely 



