144 



IN VERTEBRA TE MORPHOL OGT. 



into a tubular oesophagus whose walls are thickened near 

 its anterior end to form a muscular pharyngeal bulb which 

 functions as a pump for the ingestion 

 of the nutritive fluids of the host. At 

 its posterior extremity the oesophagus 

 branches into two limbs which are con- 

 tinued backwards, in some cases giv- 

 ing off secondary branches, to near 

 the posterior end of the body, where 

 they either end blindly or unite together 

 in the middle line (Polystomum) to form 

 a loop. 



The body is covered by a distinct 

 cuticle secreted by the ectodermal cells, 

 which in the adult may undergo a con- 

 siderable amount of degeneration, or 

 probably in some cases the cuticle is 

 formed in part by the transformation 

 into chitin of the ectoderm. Spiny ele- 

 vations of the cuticle are present in 

 many forms, and the large chitinous 

 hooks which occur in many ectopara- 

 sitic forms are but further developments of these structures. 

 Below the ectoderm lies the usually thin basement-membrane, 

 below which again lie the circular and longitudinal peripheral 

 muscle-sheets, and between the intestine and these muscles is 

 the parenchyma traversed by dorsoventral muscle-bundles 

 and having imbedded in it the various organs. 



The nervous system (Fig. 77) consists of a transversely 

 elongated ganglion lying dorsal to the oesophagus — usually 

 between the bottom of the anterior sucker and the pharyngeal 

 bulb. The ganglion is somewhat swollen at each extremity, 

 indicating its origin by the approximation of two ganglionic 

 masses, and from these thickenings nerves arise which pass 

 both forward and backward. The anterior nerves are short 

 and slender, and supply the musculature of the anterior 

 sucker and the sides of the anterior end of the body, while 

 the posterior nerves are much stronger and longer and vary 

 from two to six in number ; in the latter case four run along 



Fig. 73. — Bisiomwm cyg- 

 noides (from a drawing by 

 C. Lanoenbkck). 



