THE DEVELOPMENT OF THE XEMATOIDEA. 549 



The female reproductive apparatus is, at first, represented 

 by a solid cellular body which lies in the mesoderm ; though 

 whether it originally belongs to this, or to the ectoderm, or 

 to the endoderm, is not clear. The cellular body acquires a 

 tubular form, and eventually opens externally by uniting 

 with an inward process of the ectoderm, which gives rise to 

 the vagina. 



The young cast their cuticle twice — first, when they leave 

 the egg, and, again, when they acquire their sexual organs. 



The Keniatoidea have been divided into three principal 

 groups ^ — Polymyaria^ 3Ieromyarla^ and Ilolomyaria — char- 

 acterized by the nature of their muscular system. 



In the Poly my aria, the muscles of the parietes of the 

 body are divided into many series, each made up of many 

 '* muscle-cells." In the Meromyaria there are only eight 

 longitudinal series of such muscle-cells, two between each 

 lateral area and the dorsal and ventral lines respectively. In 

 the Ilolomyaria the muscles are not divided into series of 

 muscle-cells. 



The first two divisions contain only such genera as an- 

 swer to the general description just given ; but, in the Holo- 

 myaria, there are included several aberrant forms. Thus, 

 Trichocephalus has no lateral areas ; Ichthyonema has no 

 anus ; Mermis has no anus, and the alimentary canal is rudi- 

 mentary, though it possesses the lateral areas, and the males 

 have spicula. Gordins has no lateral areas, and onl}' the 

 ventral line ; the alimentary canal is reduced to a rudiment, 

 without either oral or anal aperture, and the male has no 

 spicula. In both these genera the anterior ends of the em- 

 bryos are provided with spines, which aid them to bore their 

 way into the bodies of the insects on which they are para- 

 sitic. In Sphmrularia the alimentary canal is similarly rudi- 

 mentary, and Sir John I.ubbock discovered that the small 

 male becomes permanently adherent to the female. 



Some Nematoidea (e. g., Leptodera, Pelodera) Hve in 

 water or damp earth, and are never actually parasitic; but 

 the}' require abundant nitrogenous food in order to develop 

 their sexual organs, and hence they are found in the sexual 



1 Schneider, " Mono^raphie der Nematoden," 1866. See also Bastian, 

 *' Monoirrapli of the Aneruillulidw" (" Trans. Linnfean Societv," 1865"^; and, 

 " On the Anatomy and Phvsioloary of the Nematoids " (" Phil. Trans.," 1866) ; 

 and several memoirs by B'ltschli. The latter affirms that the muscles are as 

 much made up of muscle-cells in the HoLomyaria^ as in the rest. (" Giebt es 

 Holomyarier ? " Zeitschrift fvr wiss. Zoologie^ 1873.) 



