i8o Introduction to Animal Morphology. 



of large,* flattish, spindle-shaped, longitudinal fibres, 

 either continuous (Trichocephalus) or divided into 

 dorsal, ventral, and lateral tracts, separated by 

 thickenings and prolongation of the cutis. Some- 

 times vesicular processes are appended to the fibres 

 (tracheal sacs of Boj'anus), and in some the processes 

 of the muscular fibres unite vvith the nerves, or run 

 into the neurilemma, especially around the pharyngeal 

 ring, reminding us of the neuro-muscular cells of 

 Hydra. The muscle fibres may be solid, flat (Platy- 

 myaria), or tubular (Coelomyaria), with a striated wall 

 enclosing granular protoplasm, continuous with that 

 in the vesicular swellings. 



The nervous system consists of an oesophageal 

 ring covered by multipolar ganglion cells, surrounded 

 by a sheath, sending forwards two branches in the 

 lateral field, and four in the mesial line, some of which 

 go to the oral papillae ; ganglion cells lie in the course 

 of these. Behind, the ring gives dorsal and ventral 

 branches ; two of the latter often speedily unite to 

 form a ventral (cephalic) ganglion, and in some young 

 forms post anal ganglionic enlargements exist. Touch 

 papillae exist in some ; perhaps the oral lobes may be 

 of this nature. Nerveless eye specks exist in some 

 free forms, while true eyes are present in the free 

 genera, Enoplus and Urolabes. 



The mouth is anterior, surrounded by lips or pa- 

 pillae, which sometimes become chitinized as jaws ; 

 it is rarely cup-like (Cucullanus). The oesophagus is 

 narrow, often gradually thickening, surrounded by 

 circular fibres in some (Heterakis, Oxyuris), separate 



* In Strongylus gigas each is 4 vun. ; Ascaris lumbricoides 3 7nm. ; 

 Dochmius hypostoma 2 mm. 



