PHYLUM ANNELIDA 



271 



Figure 15.3. Lateral view of the head of Nereis with the pharynx withdrawn (left) 

 and everted (right). 



lying directly over the brain. Each eye is a cup of modified epidermal 

 cells, the ends of which extend through a black pigment layer to form a 

 retinal lining of light-sensitive rods. The cavity is filled with a lens, 

 protruding from the cup as a spherical swelling covered by a transparent 

 layer of skin, the cornea. The eyes are directed upward and outward, 

 and are probably defensive in function, warning Xereis when a fish or 

 other large predator approaches from above. Behind the eyes are a pair 

 of small ciliated pits believed to function as chemoreceptors. 



The prostomium of Lumbricus (Figs. 15.1 and 15.4) lacks special 

 sense organs and appendages. It is richly supplied with nerve endings for 

 touch and chemoreception, and is used as a muscular probe in burrow- 

 ing. Althougli Lumbricus lacks eyes it responds to light, generally moving 

 away from it. Certain large epidermal cells scattered over the back and 

 sides of the body have been shown to be sensitive to light. 



The peristomium of Nereis (Fig. 15.3) is actually two segments 

 fused together. Four pairs of tentacular cirri, used as tactile organs, are 

 located at its anterior margin. The uppermost are the longest, and they 

 are longer in males than in females. The peristomium of Lumbricus 

 lacks appendages. 



The body may be divided into as many as 200 segments in Nereis, 



, T , Recepla.ck por<z.S 



Clifcellu:m 30 Lateral chae-t^c w Prostoipium 



J I / I \\ I 



' Peristomium. 



F-iCf-retoru- Ventral chaeta^ / -r'\ i _^^„ 



■porS ■ Maleioore^ Female pore 



Figure 15.4. Lateral view of the anterior 40 segments of Lumbricus. Reproductive 

 openings are found on segments 9, 10, 14 and 15. On each segment the excretory pore is 

 either ventral, near the ventral chaetae, or lateral, above the lateral chaetae, with much 

 variability between worms. 



