238 



UNSEGMENTED WORMS 



Invertebrate. Here and there on the ring and on the 

 nerves there are ganghonic cells, but there is but little 



aggregation of these into ganglia. Sense 

 organs are represented by the papillae 

 already mentioned. 



As the food consists of juices from 

 a living host, it is not surprising to find 

 that the alimentary canal has but a 

 narrow cavity. It consists of three 

 parts — a fore-gut or oesophagus, lined 

 by the inturned cuticle, a mid-gut or 

 mesenteron of endodermic origin, and 

 a usually short hind-gut or rectum lined 

 by the cuticle. When the external 

 cuticle is shed, so is that of the fore- 

 gut and hind-gut (cf. Crayfish). 



There is a distinct space between gut and 

 body wall, but it is lined externally by the 

 muscle cells, internally by the endoderm of the 

 gut, which has no mesoblastic coat ; the space 

 is therefore not strictly coelomic. It contains 

 a clear fluid, which probably discharges some of 

 the functions of blood. There are no free 

 amoeboid phagocytes. 



Embedded in each lateral line there is a 

 longitudinal canal. These unite anteriorly, 

 and open in a ventral excretory pore near the 

 head. They seem to be associated internally 

 with fixed phagocytic cells. In the species dis- 

 cussed there are four giant branched cells 

 situated anteriorly, which are especially con- 

 nected with taking up waste particles. The 

 relation of this excretory system to that of 

 other Invertebrates is unknown. 



The sexes are separate. In the male 

 the testis is unpaired — a coiled tube 



Fig. 130. — Diagram 

 of the structure of a 

 male Nematode. 



M., Mouth; CE., oesopha- 

 gus; GA., nerve ring; 

 B., bulb at lower end of 

 fore-gut ; G., mesen- . _p - . . 



teron ; SP., spme with gradually difterentiatmg mto vas de- 

 ejSuiatory 'duett Vs. "i fcrcus, scmiual vcsiclc, and ejaculatory 

 seminal vesicle;' T.', duct. The genital apcrturc is close to 



testis ' £, i , ion*^itucli- O 1 



nai excretory tube, cut the auus. The spcrmatozoa have not 

 short; EP., excretory ^^^ typical form, and are sluggish. 



Their movement within the female 

 ducts appears to be due to flagella-like villous processes 

 from the walls. In the female the ovary is a paired tube, 



pore. 



