302 



ZOOLOGY 



SECT. 



be 



part by comparatively thin partitions of parenchyma sometimes 

 termed mesenteries. The median ventral canal (Fig. 250, m.v.c.) 

 of the body-cavity encloses the alimentary canal: the lateral 

 canals (I.e.) are associated with the development of the reproductive 

 apparatus. In the case of the lateral alone, and only in the female, is 

 there an epithelial lining. In Nectonema the 

 body-cavity is reduced in the female to a 

 narrow space between the wall of the ovary 

 and the muscular layer : in the male it is 

 considerably wider. 



In the Nematoidea when definite excre- 

 tory organs exist, they take the form of 

 narrow longitudinal canals similar to those 

 described as occurring in Ascaris each 

 canal being an excavation in a single enor- 

 mously elongated cell, and the system 

 having a median ventral opening. Some- 

 times only one of the canals is developed. 

 Flame-cells are never present. In the Nema- 

 tomorpha such a system does not occur. 



In the Nematoidea the nervous system 

 has the structure already described in 

 Ascaris; it is, however, apparently absent 

 in some free-living forms. But in Gordius 

 ,df it is much more highly developed : the 

 pharyngeal ring (brain) is of great thickness 

 and is continued into a single ventral cord 

 composed of three strands containing 

 nerve-cells. In Nectonema there is a similar 

 ring and three-stranded nerve-cord, and in 

 addition a large anal ganglion at the pos- 

 terior end. 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 

 izzarT, and not coiled (Fig. 251, ts. and v. df.). A 

 Sis ^ ew forms are hermaphrodite, but, instead of 

 ^(From 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 pro- 

 tandrous (male products ripen first), and self -impregnation is as 

 effectually prevented as if the organs of the two sexes were distinct. 

 In Gordius and Nectonema (Nematomorpha) the sexes are 



