4 ZOOLOGY SECT. 



of a spongy character. The oUar-cavity communicates with the 

 extei-ior by a pair of coIlar-2mrs--n\mted tubes leaduig into the 

 first gill-slit or first gill-pouch. i 1,1 



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

 r,,\v of small slits— the >/ill-slUs (Fig. 70.5, &r.)— each row situated 

 m a longitudinal furrow ; these slits increase in number throughout 

 life The most anterior are in some species overlai^ped by 

 a posterior prolongation of the collar called the opcrculuvi. 



dors-V 



Fin. 700. BalanoglOBSUB. Diagrammatic Silgittal action of anterior end. ravd .s\ cardiac 



sac; '/(■'■. diverticulum (supposed notochord) ; d.ors. n. dorsal nerve-strand; d'n^. .sv';(. dors.al 

 sinus ; dor^. v. dorsal vessel ; uiri. mouth; X-^ob. proboscis ; pi-olK po. proboscis-pore ; i>rfh. sled. 

 proboscis-skeleton ; rtaf. v. ventral ncr^ c strand ; reiU. 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 

 {hrancliial 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 

 arc two rows of prominences {hep.) formed by the hepatic creca. 

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



