ZOOLOGY 



SECT. 



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

 exterior hy a pair of coUar-'puo'cs — ciliated tubes leading into the 

 first gill-slit or first gill-pouch. 



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

 row of small slits — the f/i/l-slUs (Fig. 705, hr.) — each row situated 

 in a longitudinal furrow ; these slits increase in nu'.nber throuj^hout 

 life. The most anterior are in some species overlapped by 

 a posterior j)rolongation of the collar called the opemUum. 



vent rv 

 vent V 



dcrs-V 



Fio. 700. — BalanoglOSSUS. Diagrammatic sagittal action (if anterior end. card .9. cardiac 

 sac; <}'(i\ divcrtirulum (supposed notochord) ; dors. n. dorsal nerve-strand; dorx. sin. dorsal 

 sinus ; dors. r. dorsal vessel ; mo. mouth; yi oh. proboscis ; prob. po. proboscis-pore ; prol). sl-el. 

 proboscis-skeleton ; vent. n. ventral ner^e 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 ca^ca. 

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



