8 PRIMITIVE ANIMALS 



reproductive organs, instead of lying in a packing- 

 tissue of mesoderm, are contained in a cavity full 

 of fluid. This cavity represents a blood-system but 

 it is not divided into regular vessels, and no true 

 circulatory system exists as in higher forms. 



The Phylum Nemertea, containing the Nemertine 

 worms, may perhaps be regarded as affording a link 

 between the Platyhelminth type and the next 

 phylum, the Appendiculata. The Nemertines are 

 round worms which live for the most part a free 

 life in the sea, though a few are found in fresh- 

 waters, a few live on land in damp situations, while 

 a few are parasitic. The Nemertea agree with the 

 Platyhelminths in having a packing-tissue of meso- 

 derm surrounding their internal organs, but in this 

 packing-tissue a definite circulatory system with a 

 heart is differentiated. The body in general is un- 

 segmented as in Platyhelminths, but the reproductive 

 organs consist of laterally paired hollow pouches 

 arranged in linear series along the body. This gives 

 the appearance of metameric segmentation, and it is 

 possible that this represents the beginning of the 

 condition of true metameric segmentation found in 

 the next phylum, the Appendiculata. It must be 

 noted, however, that there is a wide gap between the 

 Nemertea and Appendiculata in the matter of meta- 

 meric segmentation, for whereas in the Nemertea 

 only the reproductive pouches are involved in the 



