110 INVERTEBRATE ZOOLOGY 



logical studies ho found that the head end of the worm has the 

 highest rate of metabolism. This rate tends to decrease in the 

 region behind the head. The term axial gradient has been 

 applied to this physiological differentiation of regions along the 

 axis of the body. When a certain level is reached (Fig. 61) the 

 rate of metabolism shows a sudden increase. This point on 

 the axis of the body coincides with the region where fission would 

 first take place, and the sudden increase in rate of metabolism 

 marks the location of a new incipient head. Behind this point 

 Dr. Child found in long individuals a series of regions showing 

 alternation between regions of increased and diminishing rates 

 of metabolism. Each rise in metabolic rate marks the location 

 of a potential new head. 



The Orders. — A pharynx and blind intestine comprise the 

 digestive tract of the turbellarians. The muscular pharynx is 

 frequently enclosed in a pocket within the body and in feeding 

 is thrust out like a proboscis. 



Upon the nature of the intestine the three orders are distin- 

 guishable. Among the Polycladidea (Fig. 60 B), the mesenteron 

 consists of a central space from which numerous branches pass 

 into the parenchyma. These branches become greatly sub- 

 divided and frequently anastomose. In the Tricladidea, three 

 main branches (Fig. 60 C) lead off from the pharynx, one directed 

 anteriorly and two posteriorly. Each of these has numerous 

 lateral diverticula which nearly reach the margins of the body. 

 In the Rhabdocoelida, the mesenteron (Fig. 60 A) forms a simple 

 sac-shaped or rodlike intestine. The so-called Acoela lack a 

 cavity in the mesenteron which has been described as a digestive 

 parenchyma. 



Nervous System. — A simple, linear type of central nervous 

 system is characteristic. In the Polycladidea, there is a consid- 

 erable plexus of nerve branches with a brain near the anterior 

 extremity and a few main trunks. The triclad central nervous 

 system is essentially a pair of lateral nerve trunks which pass 

 posteriorly from the brain with irregular transverse commissures. 

 In some forms (Gunda), these connecting branches are so regular 

 that they constitute a distinctly ladder type of system. The 

 rhabdocoel nervous system is distinctly similar to that of the 

 triclad type. Highly specialized sensory organs are not common 

 in the Turbellaria. Eyes of a simple type are usually found 

 above the brain. In the polyclads, these occur in large numbers 



