PHYLUM PLATYHELMINTHES 



163 



adult appears after metamorphosis. In some forms there is a creep- 

 ing larva known as Desor's larva. The vascular system is composed 

 of longitudinal vessels connected by transverse loops. The vascular 

 fluid is usually colorless. The excretory system includes the usual 

 longitudinal tubules and flame cells characteristic of the phylum. 

 Either one pore or several communicate with the exterior. The cen- 

 tral nervous system consists of two ganglia and three longitudinal 

 cords passing through the body. A pair of grooves with cilia along 

 each side of the cephalic portion are sensory and are called cerebral 

 organs. Other tactile organs and eyes are usually developed. Pros- 

 toma, Cerebratulus, Tetrastemma are representatives. 



Georaqe cavity 



L'lji^^i^— -Mesenchymal cell 



Stomach 



Lctodermal / 



invagination 



EsophacjUi 



.fctodermaf 

 invagination 



Ventrolateral lobe 



Fig. 80. — Structure of pilidium larva of the nemertlne worm in partial section. 

 (Redrawn and modifled from Wolcott, Animal Biology, published by McGraw-Hill 

 Book Company, Inc.) 



PLANARIA 



Habitat ajid Behavior 



This free-living, fresh-water, flatworm thrives beneath the rocks, 

 logs, leaves, algae, or debris at the bottom of shallow spring-fed 

 brooks and pools. They must have pure, clear, cool water. These 



