CHAPTER XVI. 



NEMATELMINTHES AND ACANTHOCEPHALA. 



NEM ATELMINTHES l . 



Nematoidea. Although the ova of various Nematodes have 

 formed some of the earliest, as well as the most frequent objects 

 of embryological observation, their development is still but very 

 imperfectly known. Both viviparous and oviparous forms are 

 common, and in the case of the oviparous forms the eggs are 

 usually enveloped in a hard shell. The segmentation is total 

 and nearly regular, though the two first segments are often 

 unequal. The relation of the segmentation spheres to the 

 germinal layers is however only satisfactorily established (through 

 the researches of Biitschli (No. 383)) in the case of Cucullanus 

 elegans, a form parasitic in the Perch 2 . 



The early development of this embryo takes place within 

 the body of the parent, and the egg is enveloped in a delicate 

 membrane. After the completion of the early stages of seg- 

 mentation the embryo acquires the form of a thin flat plate 

 composed of two layers of cells (fig. 166 A and B). The two 

 layers of this plate give rise respectively to the epiblast and 

 hypoblast, and at a certain stage the hypoblastic layer ceases to 



1 The following classification of the Nematoda is employed in this chapter: 



r Ascaridre. 

 Strongylidte. 



Trichinidre. II. Gordioidea. 



)a - j Fiiarids. in. Chffitosomoidea. 



Mermithidae. 

 [_ Anguillulidse. 



The ova of Anguillula aceti are stated by Hallez to undergo a similar develop- 

 ment to those of Cucullanus. 



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