ANNELIDA : THE EARTHWORM, NEREIS 233 



is a flat, hollow, vertical process of the body-wall, standing 

 out at the side of its segment and serving to pull the animal 

 along in creeping, or to row it in swimming. It is cleft 

 into two principal lobes, a dorsal notopodium and a ventral 

 neuropodium. Each of these is again divided into smaller 

 lobes and bears at its base a slender process known as a 

 cirrus. A stout, deeply embedded seta or aciculum supports 

 the notopodium and another the neuropodium. The front 

 end of the body is modified to form a definite head. 

 This consists of the prostomium and the peristomium. On 

 the prostomium are situated dorsally a pair of prostomial 

 tentacles and two pairs 



of eyes, each of which flE*—^ 



is a pit lined by pig- 

 mented cells and en- 

 closing a gelatinous 

 mass which serves as 

 a lens. Ventrally the 

 prostomium . bears a 

 pair of stout palps. 

 The peristomium 

 carries on each side 

 two pairs of long, 

 slender tentacular 

 cirri, and probably 



Corresponds tO tWO ap.c., Apical tuft of cilia; eye; m., opening of 



fiic^H wompnts A mouth ; mes., mesoderm ; £r., preoral ring of 



luscu acgnicLita. -ri cilia; stm.-, stomodseum (the pouch of ectoderm 



bilaterally Symmetri- which forms the mouth and gullet). 



cal animal which 



leads an active life always has a head, and if the animal 

 be segmented there is a tendency for the foremost seg- 

 ments to enter into the composition of the head. This 

 is known as cephalisation. Behind the last segment is a 

 conical region without parapodia which bears a pair of 

 slender anal cirri and the terminal anus. The alimentary 

 canal of Nereis is simpler than that of the earthworm, 

 but the pharynx can be caused to protrude by being 

 turned inside out, and is lined with cuticle, thickened 

 in places to form numerous small teeth and a pair of 

 strong jaws with which the prey is seized. The sexes 

 are separate. The reproductive organs are very simple, 



FIG. 149. — The trochosphere of Nereis. 

 —Modified, after Wilson. 



