Phyla Platyhelminthes and Nemertina 421 



The proper disposal of sewage, the careful cooking of food, and the 

 avoidance of raw vegetables all seem comparatively simple. However, 

 carelessness, ignorance, and eating habits are hard things to change, 

 and no doubt the life cycles shall continue in the age-old manner. 



THE PHYLUM NEMERTINA 



The members of this phylum are flattened, slender worms, vary- 

 ing in length from a few millimeters to more than 60 feet. Due to 

 their flattened appearance, they are commonly called ribbon worms. 

 Most of the species are free-living, marine forms although there are 

 a few fresh- water species. A few are parasitic on marine invertebrates. 

 In the ocean they occur from the intertidal zone to deep water, where 

 they live in the bottom material and feed on smaller invertebrates. 



SENSORY OnOAN. 



Fig. 138. — The nemertine Cerebratulus. A, Anterior end; B, pilidium larva. 



The Characteristics of the Phylum. — Like the platyhelminthes, 

 these worms are without a body cavity and have the spaces between 

 the body wall and intestine filled with parenchymatous tissue. Also 

 like the flatworms they possess a prctonephridial system with flame 

 cells, a nervous system, and sense organs of similar structure, a ciliated 

 epidermis, and remarkable powers of regeneration. Unlike the flat- 

 worms, they possess a complete digestive tract (with both mouth and 

 anus), a closed circulatory system, and an eversible proboscis dorsal 

 to the digestive tract. 



The closed circulatory system is a unique feature for an animal of 

 this grade of construction. It consists of two kinds of channels : blood 

 vessels and lacunae, spaces in the parenchyma which are lined with 

 a delicate membrane. A pair of lateral vessels along the digestive tract 



