THE BRISTLE-WORMS. 



87 



Fig. 30). 



modified segments are added to it. This is interesting, because 

 when we pass to Arthropods we shall find that the head con- 

 sists of a number of segments all firmly welded together. 



The head of Nereis varies considerably in appearance, 

 according to the condition of the parts, whether fully pro- 

 truded or retracted. If a large Nereis be killed suddenly, 

 as by immersion in spirits, it will be observed to rapidly 

 protrude a large "proboscis" or "introvert," which when 

 completely everted shows at its tip a pair of powerful horny 

 jaws (see Fig. 30). The method of eversion is interesting, 

 and is one which is common among Invertebrates. It is 

 best understood by taking a glove, fastening two pieces of 

 thread about an inch from the tip of one of the fingers 

 to represent the jaws, and then doubling in the finger into 

 the glove as far as it will go (see the upper diagram in 

 The hole left where the finger is doubled in 

 represents the mouth of the 

 worm, and it will be seen that 

 the little tags representing the 

 jaws (j in Fig. 30) now lie well 

 within the mouth-cavity (m in 

 Fig. 30) ; they are not visible 

 in the worm under ordinary 

 conditions. Now carefully 

 double the inturned glove finger 

 outwards until the "jaws" lie 

 just at the tip of the part 

 turned out; this represents the 

 "proboscis" of Nereis when 

 fully everted, and then bearing 

 the jaws at its tip. All the 

 part which can be thus everted 

 is called the buccal cavity. It 

 opens into the pharynx (p in 

 Fig. 30), the next part of the alimentary canal, which is 

 represented by the remainder of the glove finger, but 

 which differs from it inasmuch as it cannot be everted, or 

 turned outwards, but can merely be protruded with its 

 terminal jaws. The head, therefore, of Nereis appears 

 entirely different according to whether the buccal region is 

 retracted or everted. In the former condition the mouth 



FIG. 30. Diagrams showing the 

 way in which the proboscis is 

 everted and retracted in Nereis, 

 The upper figure (^4) shows the 

 retracted condition, the lower 

 (Z>) the everted. For letters see 

 text. After Lang. 



