374 METAMORPHOSIS. 



the original column. In other respects the development takes 

 place as in the female. 



GrOrdioidea. The ovum of Gordius undergoes a regular segmentation. 

 According to Villot (No. 391) it forms at the close of segmentation a morula, 

 which becomes two-layered by delamination. The embryo is at first 

 spherical, but soon becomes elongated. 



By an invagination at the anterior extremity the head is formed. It 

 consists of a basal portion, armed with three rings of stylets, and a conical 

 proboscis, armed with three large stylets. When the larva becomes free 

 the head becomes everted, though it remains retractile. By the time the 

 embryo is hatched a complete alimentary tract is formed with an oral opening 

 at the end of the proboscis, and a subterminal ventral anal opening. It is 

 divided into an oesophagus and stomach, and a large gland opens into it at 

 the base of the proboscis. 



The body has a number of transverse folds, which give it a ringed 

 appearance. 



Metamorphosis and life history. 



Nematoidea. Although a large number of Nematodes have 

 a free existence and simple life history, yet the greater number 

 of known genera are parasitic, and undergo a more or less com- 

 plicated metamorphosis 1 . According to this metamorphosis 

 they may be divided into two groups (which by no means 

 closely correspond with the natural divisions), viz. those which 

 have a single host, and those with two hosts. Each of these 

 main divisions may be subdivided again into two. 



In the first group with one host the simplest cases are those 

 in which the adult sexual form of parasite lays its eggs in the 

 alimentary tract of its host, and the eggs are thence transported 

 to the exterior. The embryo still in the egg, if favoured by 

 sufficient warmth and moisture, completes its development up 

 to a certain point, and, if then swallowed by an individual of the 

 species in which it is parasitic in the adult condition, it is 

 denuded of its shell by the action of the gastric juice, and 

 develops directly into the sexual form. 



Leuckart has experimentally established this metamorphosis in the case 

 of Trichocephalus affinis, Oxyurus ambigua, and Heterakis vermicularis. 

 The Oxyuridae of Blatta and Hydrophilus have a similar life history 



1 The following facts are mainly derived from Leuckart's exhaustive treatise 

 (No. 388). 



