104 ANNELIDA 



are present they offer a clue to the number of segments, since 

 there is ordinarily one pair of parapodia per segment. Where 

 parapodia are absent, internal structures (e. g., septa) may 

 help to define segments. See table on page 103. 



Internal Structure. — If fresh material is used, anesthetize 

 first in 8 per cent alcohol in sea water. Pin the specimen 

 out in a dissecting pan, dorsal side up, sticking two pins 

 through the sides of the first segment and two through the 

 sides of the tail region. Cover the specimen with water. 

 Open the worm by making an incision with a fine pair of 

 scissors in the mid-dorsal region near the center of the gill- 

 bearing portion. Before continuing with the cut, notice the 

 coelomic fluid. Examine some of this under the microscope. 

 Observe the coelomic corpuscles and possibly the reproduc- 

 tive cells. There are two kinds of coelomic corpuscles: 

 amoebocytes and fusiform cells. These cells form clots when 

 removed from the coelom. They are also phagocytic. 



Next raise the point of incision carefully with forceps and 

 extend it to within an eighth of an inch of the prostomium. 

 Posteriorly continue the cut about an inch into the tail re- 

 gion. The tail region is difficult to open and, unless you are 

 careful, you may pierce the intestine. The flaps of the body 

 wall should now be pinned out right and left so that the worm 

 is moderately well stretched both longitudinally and trans- 

 versely. At the breeding season ova or spermatids are so 

 abundant that they may obscure some of the organs. In 

 this case the reproductive cells should be carefully washed 

 away. 



Note that the septa are confined to the anterior and pos- 

 terior regions of the body. On the anteriormost septum there 

 is a pair of diaphragmatic pouches, which extend posteriorly. 

 It is possible that these may have some function in connec- 

 tion with the extension of the proboscis. What advantage 

 is there in the absence of septa throughout the principal part 

 of the worm? 



Digestive System. — The protrusible proboscis region is 



