204 



UNSEGMENTED "WORMS" 



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 discussed there are four giant branched cells situated 

 anteriorly, which are especially connected with 

 taking up waste particles. The relation of 

 this excretory system to that of other In- 

 vertebrates is unknown. 



The sexes are separate. In the 

 male the testis is unpaired a coiled 

 tube gradually differentiating into vas 

 deferens, seminal vesicle, and ejacu- 

 latory duct. The genital aperture is 

 close to the anus. The spermatozoa 

 have not the 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, which passes gradually into 

 an oviduct and a uterus at each side, 

 and a short unpaired vagina. 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 

 the formation first of a blastula and 

 then of a gastrula. The germ-cells are 

 distinguishable very early! from the 

 body-cells. Blastopore and archen- 

 teron are obliterated, the mid - gut 

 arising as a secondary splitting between 

 two rows of endoderm cells. The eggs 

 pass out of the gut of the host and 

 probably hatch in water, and are thus 

 re-introduced. No intermediate host 



FIG. 107. Diagram of 



the structure of a 



male Nematode. 



M., mouth; CE. t oesophagus; 



GA., nerve ring ; 2? M bulb 



at lowerend of fore-gut; G., 



mesenteron ; SP., spine 



seminal vesicle ; 7\, testis; IS 

 ET. t longitudinal excre- 

 tory tube, cut short ; EP. , The Nematoda are interesting both on 

 excretory pore. account of their parasitism and on account of 



their peculiarly isolated zoological 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 individuals which may infest one host shows how successful 

 the parasitism is, yet Nematodes exhibit few of the ordinary adapta- 



