NEMERTINES. 



the Echinoderms. It will next be shown that there are 

 certain features in the adult anatomy which apparently 

 indicate a distinct genetic relationship to another group of 

 the Invertebrates ; namely, the Nemertine worms. 



The Nemertines are elongated, flattened, or cylindrical 

 worms, with a smooth ciliated skin and no external seg- 

 mentation, occurring, as a rule, in a closely similar habitat 

 to that of Balanoglossus, buried in the sand or mud of the 

 sea-shore. 



Like Balanoglossus, they also possess unicellular integu- 

 mentary glands, by means of which they secrete a mucous 

 substance, to which frequently sand-grains adhere, thus 

 producing a tube of sand round the body. Some of them 

 reach an enormous length, and one at least must be 

 measured in yards (Linens longissimus exceeding three 

 yards in length). 



The chief anatomical features which offer material for 

 direct comparison between the Nemertines and Balano- 

 glossus relate to the ectoderm, proboscis, nervous system, 

 mesenchymatous tissue, the reproductive organs, and the 

 alimentary canal. 



As for the ectoderm, considered apart from the nervous 

 system, it need only be repeated that in both cases it is 

 composed of ciliated cells and scattered mucous glands. 



The proboscis of the Nemertines is one of the most 

 characteristic organs of this group of animals. It is not 

 permanently protruded, and does not serve as an organ of 

 locomotion, as in Balanoglossus, but is usually carried 

 about entirely withdrawn within the body of the animal, 

 from which it can be shot out with great force and rapidity 

 when the occasion demands it. During the process of 

 extrusion it is turned completely inside out, and conversely, 

 during the process of introversion, the retraction takes 



