272 



THE ANIMAL KINGDOM 



NEREIS 

 Doi\=;al blood, vessel 



Epidermis 

 Circular muscle 

 Lon^. rnuscle 



Sept una 



Aciculum 

 Chaetac 



BloodL vessel 5- (Labzral)nVentral)' ^Nerve cord 



Epidermi 

 Circular muscl 

 Lon6iludinal mu-Scle 

 Sc-ptum 



Chaeta prolractor. 

 Chacta- retracl;oi 



topodium 



Ncuropodium. 

 Vcnlral cirrus 

 Opening in-to next Segment 



Blood vessels : (Venlral) 



orsal bloodvessel 

 Chlora^en cells 

 Typhlosole 

 Nephridium 

 Iritcstine 

 •Opening irrbo next Serine nl 



ly^Crztory pore, 

 . , x^c-jeral nerve 



^^ Nerve coi'd 

 LUMBRICUS 



Figure 15.5. Cross sections of Nereis (above) and Lumbricus (below). Each is a seg- 

 ment viewed from in front, with the septmn behind. In Nereis, on the left side the body 

 wall has been cut back to show the internal structure of the parapodium. In Lumbricus 

 the body wall is cut at the le\el of the excretory pore on the right side, and further back 

 at the level of the chaetae on the left side. 



180 in Lumbricus. Young worms have fewer segments and apparently 

 new ones are added posteriorly throughout life. A mid-dorsal Hne indi- 

 cates the underlying dorsal blood vessel, and a mid-ventral line indicates 

 the position of the ventral nerve cord. These lines are faint in Lum- 

 bricus. Both species are more heavily pigmented above than below. 



Every body segment of Nereis except the peristomium bears a pair 

 of parapodia (Fig. 15.5), each of which is divisible into a dorsal noto- 

 podium and ventral neuropodium. Each portion has several lobes and 

 bears a tuft of many chaetae. A slender, tactile dorsal cirrus projects up 

 from the notopodium, and a ventral cirrus extends down from the base 

 of the neuropodium. The upper lobes of the notopodia are large and 

 richly vascularized, serving as gills. Internally, each tuft of chaetae clusters 



