x SEPTA ALIMENTARY CANAL 269 



Since primitively the whole' skin of the worm is respiratory, 

 any part of the skin may become more or less specialised for this 

 function, and chiefly, of course, on the more actively moving para- 

 podia. The blood-vessels constituting the essential part of the 

 " gill " may make use of any already existing outgrowth (such as 

 a cirrus or a tentacle), or may push the body- wall out on their 

 own account. 



Internal Anatomy. 



Probably those organs which have the greatest effect in modi- 

 fying the shape of the body are the septa, for we find in the long, 

 free-swimming worms that these are regularly present through- 

 out the body, and external " segmentation " of the body is well 

 marked. In burrowing and tubicolous forms the septa are fre- 

 quently incompletely developed, or more or fewer may be absent ; 

 and the body becomes less distinctly segmented externally, tends 

 to vary greatly in diameter during movement, or becomes 

 plumper. With the disappearance of the septa there is also a 

 diminution in the number of nephridia, as in Arenicola, with 

 only six pairs. Further, there is frequently a dimorphism of 

 these organs ; instead of all of them serving equally as excretory 

 organs and as genital ducts, some of the most anterior in the 

 Sabelliformia and Terebelliformia become greatly enlarged, and 

 take on practically the whole of the former function ; whilst 

 more or fewer of the posterior nephridia dwindle in size, and 

 become genital ducts. The absence of septa allows a free com- 

 munication between the successive segments, and thus a freer flow 

 of coelomic fluid for the distension of the anterior end of the 

 worm during burrowing. 



The alimentary system presents certain modifications of a 

 systematic value. In the Nereidiformia the muscular pharynx, 

 which is always protrusible and is preceded by an eversible 

 buccal region, frequently encloses thickened cuticular plates which 

 serve as crushing and grasping organs. The form, number, and 

 arrangement of these "jaws" vary in the different families. 

 They form valuable fossil records of extinct worms. 



In the Scoleciformia and Capitelliformia the buccal region 

 exists, but there are no jaws. In the Sabelliformia and Tere- 

 belliformia eversion does not take place and jaws are absent. 



