VI 



PHYLUM NEMATHELMINTHES 



307 



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In Nematoidea the body-cavity is always a single continuous 

 chamber crossed in various directions by delicate fibres, but in 

 Gordius certain partitions or mesenteries (Fig. 245, mes.) extend 

 longitudinally through it, dividing it into several compart- 

 ments. The most important of these are 

 a median ventral compartment containing 

 the intestine and the nerve-cord, a pair 

 of large lateral compartments containing 

 the ovaries, and a pair of small dorso- 

 median canals which act as oviducts. It 

 is stated that the median ventral com- 

 partment acts as an excretory canal 

 and opens posteriorly along with the ovi- 

 ducts : in the Nematomorpha there are 

 no lateral excretory canals like those of 

 Ascaris and the other typical Nematodes. 



In the Nematoidea, when definite 

 excretory organs are developed, they 

 take the form of longitudinal canals 

 similar to those described as occurring in 

 Ascaris. Sometimes only one canal is 

 present. In some cases it is stated that 

 the canal or canals open into the body- 

 cavity. 



In the Nematoidea the nervous 

 system has the structure already de- 

 scribed in Ascaris ; it is, however, 

 apparently absent in some free-living 

 forms. But in Gordius it is much more 

 highly developed : the pharyngeal ring 

 is of great thickness and is continued 

 into a single ventral cord (Fig. 245, bm.) 

 containing nerve-cells. Eye-spots have 

 been described in the sexual form of 

 Gordius. 



The reproductive organs in all the 

 Nematoidea resemble those of Ascaris, the 

 only important variation depending upon 

 the fact that in the smaller forms the 

 entire genital tube (gonad plus gonoduct) 

 is short and not coiled (Fig. 246, ts. and 

 v. df.). A few forms are hermaphrodite, 



but, instead of having a double set of reproductive organs, 

 as in Platyhelminthes, organs of the ordinary female nematode- 

 type are present, and the gonads produce first sperms and 

 afterwards ova. Such animals are said to be protandrous (male 

 products ripe first), and self-impregnation is as effectually 



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Oxyuris, from the 

 right side. gz. gizzard ; int. 

 intestine ; ph. pharynx 

 pn. s. penial seta: ; ts. testis ; 

 v. df. vas deferens. (From 

 Shipley, after Galeb.) 



