VIII] ANATOMY 129 



proctodaeum with muscular walls, and this terminates in the 

 anus situated a little in front of the end of the tail. 



The nervous system consists of a ring round the oesophagus, 

 which sends off in front six nerves to the mouth and its papillae, 

 while behind it also gives off six nerves, the two more important of 

 which run down the body in the above-mentioned median dorsal 

 and ventral ectodermic ridges. Transverse commissures unite these 

 main nerve- trunks at irregular intervals. With the exception of 

 certain hairs and papillae to which a tactile sense has been ascribed, 

 and in the free-living species certain eye-spots, no organs of sense 

 are known in the Nematoda. The excretory function is usually 

 assigned to two long tubes which run along the lateral thickenings 

 of the ectoderm. These tubes end blindly behind, but anteriorly 

 the tubes approach one another ventrally and open by a common 

 pore in the middle ventral line some little distance behind the 

 mouth (Fig. 54). Each of these tubes is stated to consist of one 

 immensely elongated hollow cell. Each is probably to be regarded 

 as a degenerate nephridium. 



Nematoda with few exceptions have the sexes separated. The 

 male is often smaller than the female and frequently has a curved 

 tail. In both sexes the reproductive organs are tubular. In the 

 male the organ consists of a long tapering tube much folded on 

 itself opening into the proctodaeum close to the anus. In the 

 uppermost and narrowest part of the tube there is a mass of proto- 

 plasm with nuclei; lower down the mother-cells of the male cells 

 become separated, while the lowest part contains ripe male cells. 

 These male cells are not spermatozoa however, but small oval cells 

 with large nuclei capable of only very sluggish amoeboid movement. 

 The names testis and vas deferens are given to the upper part and 

 lower part respectively of this organ, but the whole is one continuous 

 structure developed from a single cell in the embryo. In the female 

 there are two similar tubes which unite to open in the mid-ventral 

 line by an exceedingly short median piece termed the vagina. 

 The vagina is situated about one-third of the body-length from the 

 head. In each tube it is usual to distinguish an upper ovary con- 

 sisting of a mass of nucleated protoplasm, a middle oviduct where 

 the bodies of the egg-cells have become separated from one another, 

 and lastly a uterus where the eggs after fertilisation are each pro- 

 vided with a shell. Each tube however, like the testis of the male, 

 is developed in the embryo from a single cell. 



In order to introduce the motionless male cells the male distends 

 s. & M. 9 



