V. Flatworms 



TURBELLARIA. Among the turbellarian flatworms those of the 

 Rhobdocoelida are the simplest. Bohming. 1890, describes and 

 figures a number of central nervous systems from Alioeocoeia such 

 as shown in Fig. 8. The ganglia are somewhat concentrated but 

 show right and left halves. Two or four pigment spots imbedded in 

 the brain substance may show but little indication of differentiation 

 into eyes. 



Among the Acoela some have simple pigment spots for eyes 

 and some are without them. Statocysts are found in the center of 

 the ganglionic masses in some cases. Very often a well-marked 

 statocyst or otocyst may be seen in the center of the upper portion 

 of the animal, just between the pigment spots when they are pres- 

 ent. The brain is not very extensive in Acoela. It is usually recog- 

 nized as a small mass of cells surrounding the central statocyst. 

 Lohner in Pvlyclioenis gives about as complete account of the 

 nervous system as any. There is a central ganglion with a central 

 otocyst. Laterally there are two ganglia of nearly equal size. These 

 ganglia in cross section are nearly central in position while the 

 peripheral nervous system consists of longitudinal strands both 

 dorsal, ventral and lateral in position. Figure 8 shows the plan of 

 the nervous system as a whole. 



De Quatrefages, 1884, and Peebles, 1915, and others give some 

 indications of the nervous system and .sense organs of these worms, 

 but not much in detail. 



Many investigators have dealt with the Rhabdocoela. The 

 brain is a little more complex than that of the other groups men- 

 tioned but the whole s.vstem is compact and there are few longi- 

 tudinal cords from the brain region. 



Some forms have from two to four simple eyes imbedded in the 

 brain. Sensory pits near the head end are found connected with 

 the brain in .some. Ott, '92, describes "dish-shaped" organs near 

 the dorsal surface of the body of Stenostoma. In this form the 

 ciliated pits are imbedded in the forward portions of the brain. In 

 other forms, thev seem to be entirely separate. 



Schneider. '7.3, finds the lobes of the brain connected by a 

 double com'ni.ssure which surrounds the vascular system. Hallez, 

 "79, Ott, '92. and others find but a single commissure. 



The fibrous jxjrtion of the brain or "punkt substance" is com- 

 posed of a fine network of fibers which some have thought was 

 made up of ana.stomosing processes, but the evidence is not clear. 

 Nansen, '87. does not believe in an anastomosis. 



Some of the figures from the nervous systems of this group 

 show few branches. Probably more branches were present although 

 not recognized in ever.v case by the investigators. 



