BRISTLE-WORMS 



427 



Digestive organs. On cutting open the body of Nereis we 

 find that it is traversed by a straight food-tube, between which 

 and the body-wall is a large body-cavity which corresponds with 

 that of Vertebrates in being a coelom, i.e. a body-cavity which 

 contains a lymph-like fluid and communicates with the exterior 

 by excretory tubes (see p. 428). The segmentation prominent 

 externally is emphasized internally by the division of the body 

 into a series of compartments by transverse partitions attached 



A. 



RAIN 



Fig. 265. Structure of Sea-Centipede (Nereis], enlarged to various scales 



A, Head, with mouth-cavity everted and jaws protruding. B, A foot-stump (parapodium). c, Front part of 



nervous system. 



to the body-wall at the places where grooves mark off the seg- 

 ments from one another. These partitions are also attached 

 to the gut, and keep it in place, thus acting like the folds known 

 as mesentery in Vertebrates. The food-tube or gut consists suc- 

 cessively of mouth -cavity, pharynx, gullet, and intestine. An 

 interesting peculiarity of the short mouth-cavity is found in the 

 fact that it can be everted, or turned inside out, by means of 

 appropriate muscles, and under those circumstances the absence 

 of limb-jaws is made up for by the protrusion of a pair of hard 

 horny jaws carried on the inner side of the pharynx and acting 

 as a very efficient pair of pincers for seizing food. The everted 

 mouth-cavity is restored to its normal position by certain muscles 

 attached to its wall which draw it back. The pharynx has ex- 

 tremely thick walls, another point in which one is reminded of 

 Peripatus (see p. 401). The gullet is short and narrow, and 

 a pair of glands open into it. The intestine, which makes up 

 the greater part of the gut, is thin-walled. 



Circulatory Organs. We can here distinguish, as in a Verte- 



