ASCARIS. 



191 



-Of 



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 ccelomic. It 

 contains a clear fluid, which probably dis- 

 charges some of the functions of blood. 

 There are no free cunceboid 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 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 

 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 eggs pass out 

 of the gut of the host and probably 

 hatch in water, and are thus re-intro- 

 duced. No intermediate host has yet 



genital 



F I G. 92. Illustrating 

 the structure of a Ne- 

 matode (Oxyuris). 

 After Galeb. 



///., Mouth ; c., a cuticular 

 ring ; a?., oesophagus ; B., 

 bulb containing teeth ; z., 

 intestine; T., testis; i'.d., 

 vas deferens; sfi., penial 

 spine at anus. 



been found. 



The Nematoda form an important group, interesting both on account 

 of their parasitism and on account of their peculiarly isolated zoological 

 position. Though parasitism is exceedingly common, many are free- 



