158 PLATHELMINTHES 



branches open to the outside. The excretory system consists of a median 

 canal or one to four pairs of longitudinal canals which open to the 

 outside through usually paired pores and numerous small canals which 

 penetrate the parenchyma in all directions and end in flame cells. The 

 nervous system consists of a pair of ventral nerve cords, the anterior 

 ends of which are enlarged to form a brain and which are joined by 

 numerous transverse commissures. From the brain nerves go to the eyes 

 and tentacles, when these organs are present, and also to the sensitive 

 anterior end of the body. 



The reproductive organs are very complex, the animals being with 

 rare exceptions hermaphroditic, and differ somewhat in the various groups. 

 The genital opening, which is either single or double, is in the ventral 

 surface back of the mouth. Most species lay their eggs in capsules 

 which are attached to plants or stones. A few reproduce also asexually, 

 by transverse fission. 



Habits and Distribution. Most turbellarians are aquatic animals, 

 living either in fresh or in salt water; only the Terricola are terrestrial. 

 The largest aquatic form (Leptoplana gigas) may be 15 cm. in length, 

 while the largest land turbellarians (Bipaliidae) may be 45 cm. long; the 

 smallest forms are of microscopic size. They are with few exceptions 

 carnivorous animals, living on small animals of all sorts: a few are 

 parasitic. 



History. 0. F. Miiller in 1776 first separated the turbellarians and 

 nemerteans from the other flatworms and placed them in the genus Pla- 

 naria. Ehrenberg in 1831 named the group Turbellaria. In 1851 Vogt 

 placed it with the other flatworms in the class Platelmia. The present 

 arrangement of the group is due principally to von Graff and Lang. 

 About 1,100 species of turbellarians are known, grouped in 2 subclasses. 



Key to the subclasses of Turbellaria: 



QI Minute marine forms without intestine 1. ACCELA 



o 2 Intestine present 2. CGELATA 



SUBCLASS 1. ACCELA.* 



Small, delicate marine turbellarians which are found free-swimming 

 and also among the rocks and seaweed along the shore. They are often 

 brightly colored, and in one genus at least (Convoluta) the pigment is 

 due to a symbiotic alga. No intestine is present, the food, which consists 

 of minute animals and plants, being introduced directly into the paren- 

 chyma. The mouth is often near the front end of the body, a proboscis 

 being often absent. Eyes are usually absent, but a statocyst lies over the 

 brain. The reproductive system is simple in structure, in many species 



* See "Turbellaria, I. Acoela," by L. von Graff, Das Tierreich, 1905. 



