VERMES 



117 



D 



respects they are more highly developed, so that they deserve a 

 subtype for themselves. Like the Turbellaria, the Nemertina 



are free-living, and the body is soft and covered with cilia ; it is 

 always elongated and sometimes greatly so (Figs. 107 and 108). 

 These worms for the most part are marine, though some are 

 found in fresh water and in moist places on land. The mouth 

 is near the anterior end of the body 

 on the ventral side and leads into 

 a straight alimentary canal which 

 terminates in an anus at the pos- 

 terior end. This is the first group 

 of worms in which an anal opening 

 is present. There is further, at 

 the anterior end, usually on the 

 dorsal side, an opening through 

 which a long tube, the proboscis, 

 may be protruded ( Fig. 109). This 

 structure is characteristic of the 

 Nemertina. It lies within the body 

 in a sheath on the dorsal side of 

 the alimentary canal and has a 

 band of muscle fibers attached to 

 its deeper end, by means of which 

 it may be drawn back into the body 

 after it has been protruded. When 

 projected, it is turned inside out, 

 and in some cases the free ex- 

 tremity is armed with one or more 

 spines or stylets, so that it may 

 serve as an organ of defense; in FlG - 10 ' 



a • Nemertina found on the coasts ot Great 



SOme Cases it may be poisonous, Britain. (After Macintosh.) 



and it may have a tactile function. 



In the Nemertina, again, we find, for the first time among the 

 worms, a distinct blood system, which consists of three principal 

 longitudinal vessels, two lateral and one dorsal, and these are 

 connected at the anterior and posterior ends and give off 

 numerous lateral branches. The blood is usually colorless, 

 though in some cases red. It is, then, the presence of these 

 three organs, the anus, the proboscis, and the blood system, that 



