■•218 



PARASITES OF THE DO.MESTIC ANIMALS 



a,iius the alimoiitan' canal is usually a uniform tube with little or 

 no flexion. 



The nervous system consists of a nerve ring surrounding the esophagus, 

 and of the nerves given off from this ring passing forward and I^ack, 

 the largest of which are in the dorsal and ventral lines. Along the course 

 of these nerves there are ganglionic cells, ])ut there is no massing to form 

 true ganglia as occurs in the Annelida. 



The sexes are usually separate, hermaphroditic forms occurring 

 among free-living species. In general, the females attain a distinctly 



greater length and thickness of 

 the 1 )0(1}' than do the males, and 

 in other respects thej^ can easily 

 he distinguished. The males are 

 usually provided with chitinous 

 cojiulatorj' organs known as 

 spicules. These are curved, 

 spine-like structures which lie 

 in a sheath close to the anus, 

 and they can Ije protruded or 

 retracted through the cloacal 

 opening (Fig. 120). They are 

 usually two in number, but 

 there may be but one. The 

 character of the spicules often 

 serves as a guide in the estab- 

 lishment of relationships of cer- 

 tain groups. Surrounding the 

 spicules there is, in some forms, 

 a membraneous expansion which 

 is referred to as the caudal bursa 

 or pouch. This structure is best 

 developed in the Strongylidse, 

 where its varied characteristics 

 furnish an aid in the recognition 

 of species. The bursa is a clasp- 

 ing organ used in copulation, while the spicules serve to direct the course 

 of the semen. In the female there is a special genital opening, — the 

 vulva, located on the ventral anterior half of the body, or it may be to- 

 ward the anus, its position varying according to species. The cylindrical 

 bod}' is usually more or less distended with eggs, and frequently' the 

 egg-packed uteri can be distinctly seen under low magnification and 

 transmitted light. 



The internal I'cproductive systems of the male and female are nruch 

 alike. In both thev are long tubular organs, coiled forward and l)ack, 



Fig. 120. — Posterior exti'cmity of male nema- 

 tode; diagrammatic longitudinal section: el, 

 cloaca; d, intestine; m, retractor muscle of 

 spicule: s, sheath of spicule; w, body-wall (after 

 Boas, by Kirkaldy and Pollard). 



