NEMATELMINTHES. 309 



Ail inner layer is then formed by delamination. What the inner layer 

 gives rise to is not clear, since the whole alimentary canal is stated to be 

 derived from two buds, which arise at opposite ends of the body, and 

 grow inwards till they meet. 



The generative organs. The study of the development of the 

 generative organs of Nematodes has led to some interesting results. 

 In the case of both sexes the generative organs originate (Schneider, 

 No. 390) from a single cell. This cell elongates and its nuclei multiply. 

 After assuming a somewhat columnar form, it divides into (1) a 

 superficial investing layer, and (2) an axial portion. 



In the female the superficial layer is only developed distinctly in 

 the median part of the column. In the course of the further 

 development the two ends of the column become the blind ends of 

 the ovary, and the axial tissue they contain forms the germinal tissue 

 of nucleated protoplasm. The superficial layer gives rise to the 

 epithelium of the uterus and oviduct. The germinal tissue, which is 

 originally continuous, is interrupted in the middle part (where the 

 superficial layer gives rise to the uterus and oviduct), and is confined 

 to the two blind extremities of the tube. 



In the male the superficial layer, which gives rise to the epi- 

 thelium of the vas deferens, is only formed at the hinder end of the 

 original column. In other respects the development takes place as 

 in the female. 



Gordioidea- The ovum of Gordius undergoes a regular segmentation. 

 According to Villot (N"o. 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 complicated meta- 

 morphosis 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. 

 1 The following facts are mainly derived from Leuckart's exhaustive treatise (No. 388). 



