56 INVERTEBRATE ZOOLOGY 



the prosoma, or primitive head, and the metasoma, or the primitive 

 segmented trunk. The prosoma may be further - divided into 

 the prostomium, which lies in front of the mouth and contains 

 the brain and the principal organs of special sense, and the 

 metastomium, which contains the mouth. In Nereis the pro- 

 stomium bears the following special sense-organs : a pair of palps, 

 large cylindrical projections extending forward at its anterior 

 end ; a pair of tentacles, two delicate organs between the palps ; 

 and two pairs of eyes, small bead-like organs near the base of 

 the palps. Carefully identify all of these organs and notice 

 whether the palps and tentacles are jointed. The metastomium 

 is, in Nereis, fused with the first two somites of the metasoma or 

 trunk, and the segment thus formed is called the peristomium. 

 It bears the mouth and two pairs of long, flexible sense-organs 

 called the peristomial cirri. Carefully observe, with the aid of 

 a hand lens, their exact position. These cirri are morpholog- 

 ically not cephalic organs, as are the palps and the tentacles, 

 but are remnants of appendages of the first two somites. 



Exercise l. Make an outline of the dorsal aspect of the head 

 and the first five or six somites on a scale of 5. Number 

 the somites. Carefully label all the parts. 



Exercise 2. Draw a side view of the head on a scale of 5. Take 

 special care to represent accurately the position of the 

 peristomial cirri. 



Exercise 3. Find a specimen, if possible, with the proboscis 

 thrust out and draw a dorsal view of its head. 



Note the tapering of the body at the hinder end. The worm 

 grows in length at this end. The posterior somites are the 

 youngest and hence the smallest. 



Exercise 4. Make a sketch of the hinder end of the animal. 

 The long sense-organs at the extreme end are called caudal 

 cirri. In which direction do they project ? 



