PHYLUM PLATHELMINTHES 125 



functions of digestion and distribution are performed by a 

 gastrovascular system. 



Excretory System. — The excretory system consists of two 

 longitudinal tubules which run parallel to the lateral vessels of 

 the circulatory system and through their course give off small 

 branches which terminate in flame cells. Either a single pore or 

 several pores communicate with the exterior. 



Nervous System. — The central nervous system consists of a 

 pair of ganglia from which two lateral and one median dorsal 

 nerve pass backward through the body. Details of structure and 

 arrangement of the nerve trunks differ considerably in the 

 different orders. In some instances (Protonemertini), the 

 nervous system remains in the superficial layers of the body 

 external to the musculature. A pair of ciliated grooves on the 

 sides of the head, frequently called cerebral organs, are closely 

 connected with the brain and have a sensory function. Eyes 

 and tactile organs are usually developed. 



Outline of Classification 



Phylum Plathelminthes.- — Triploblastic, wormlike animals; without body 

 cavity; lacking true segmentation. 



I. Class Turbellaria. — Ciliated; chiefly free-living. 



1. Order Polycladidea. — Many branches to digestive tract; 

 marine. Planocera, Leptoplana, Stylochus. 



2. Order Tricladidea. — Three main branches to digestive tract, 

 one anterior and two posterior. Planaria, Bdelloura, Dendrocoelum. 



3. Order Rhabdocoelida. — Digestive tract a simple sac. Micro- 

 stomuni, Stenostomum, Prorhynchiis. 



II. Class Trematoda. — Parasitic; non-cellular cuticula covers body; 

 suckers for attachment; alimentary canal present. 



a. Subclass Monogenea, — Ectoparasitic; development direct; suck- 

 ers and hooks for attachment. 



1. Order Monopisthocotylea. — Posterior attachment organ single. 

 Gyrodactylus, Dactylogyrus, Nitzschia. 



2. Order Polyopisthocotylea. — Posterior attachment organ double 

 or multiple. Polysiovia, Microcotyle, Sphyranura. 



h. Subclass Digenea. — Endoparasitic; alternation of generations and 

 alternation of hosts; one or two suckers. 



1. Order Gasterostomata. — Anterior sucker imperforate; mouth 

 on midventral surface. Bucephalus. 



2. Order Prostomata. — Anterior sucker surrounds mouth which is 

 at or near anterior tip. 



a. Suborder Aspidocotylea. — Oral sucker wanting or poorly 

 developed; ventral sucker powerful disc. Aspidogaster, 

 Cotylaspis. 



