176 



UNSEGMENTED " WORMS: 



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 the absent blood. There are no amceboid phagocytes. 



Imbedded 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 in- 

 ternally with phagocytic cells. In the 

 species discussed there are four giant cells 

 situated anteriorly, which are especially con- 

 nected with taking up foreign 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 gradually differentiating into vas 

 deferens, seminal vesicles, and ejacu- 

 latory duct. The genital aperture is 

 close to the anus. The spermatozoa 

 have not the typical form, and are 

 sluggish. In the female the ovary is 

 a paired tube, which passes gradually 

 into an oviduct, a uterus, and a short 

 vagina at each side. The genital 

 aperture is ventral and anterior. 



The ova meet the spermatozoa at 

 the junction of uterus and oviduct. 

 Segmentation is total, and results in 

 £—.50 the formation first of a blastula and 



then of a gastrula. The eggs pass out 

 of the gut of the host and probably 

 Fig. 78. — Illustrating hatch in water, and are thus re-intro- 

 the structure of a Ne- duced. No intermediate host has yet 

 been found. 



matode (Oxyuris). 

 After Galeb. 



m. , Mouth; c, a cuticular 

 ring ; te. , oesophagus ; B. , 

 bulb containing teeth ; z., 

 intestine ; T., testis ; v.d., 



The Nematoda form an important group, 

 interesting both on account of their parasitism 

 . and on account of their peculiarly isolated 

 rine^anuV *" ^"'^ zoolo g ical position. Though parasitism is 

 exceedingly common, many are free living 

 for at least a part of the life-cycle, and feed 

 on putrefying organic matter. Again, although the number of indi- 

 viduals which may infest one host shows how successful the parasitism 

 is, yet Nematodes exhibit few of the ordinary adaptations to a parasitic 

 life, and there is no sharp structural line of demarcation between free 



