680 THE ANIMAL KINGDOM. 



to communicate with the mesenteron in Ichthyonema globiceps. It consists 

 of an internal cuticle, a well-developed subcuticula, and a circular 

 muscular coat, the latter derived from the radial bands which originate 

 from the body-walls, and act as divaricators. The anus is a more or less 

 transverse slit, usually opening on the ventral aspect of the tail. It is, 

 however, sometimes terminal or sub-terminal, e. g. Trichina, Trichocephalus^ 

 and the male Strongylidae, in which it is surrounded by a membranous 

 umbrella-like expansion, or bursa. The anus is wanting in Ichthyonema 

 and Mermis. Two or more unicellular glands are said to open on each 

 side of it in some cases, e. g. in Dochmius. 



An excretory system is probably always present, though it is not 

 certainly known in all the small free-living species. The excretory pore is 

 ventral and anterior near the head. A narrow canal, rarely ampullar as in 

 Oxyuris, runs backwards from it, and opens into a transverse vessel at 

 about the level of the posterior end of the oesophagus. The transverse 

 vessel is continuous on either side with a longitudinal vessel which runs 

 backwards, imbedded in the corresponding lateral area at its internal 

 margin. A vessel is also sometimes prolonged forwards on each side to- 

 wards the head, and is then either continuous with the corresponding 

 posterior vessel, or unites with a separate cross canal which opens however 

 at the same external pore. There are few variations from this typical 

 structure. The vessels end blindly ; their walls consist of a granular 

 substance with nuclei, and an internal highly refractile layer ; their fluid 

 contents are clear, and somewhat reddish, at least in the larger forms. 

 Two unicellular glands open in many instances, one on each side of the 

 excretory pore 1 . 



The Nematoda are of separate sexes with the exception of the genus 

 Angiostomum, where one generation is composed of individuals which are, 

 anatomically speaking, females, but in which spermatozoa are first of all 

 formed, then ova, and the latter are thereupon impregnated. They are, 

 therefore, physiologically hermaphrodite and self-impregnating. The 

 genital organs are tubular. The testis is single ; very rarely paired. It 

 opens on the ventral aspect of the rectum close to the anus, and therefore 

 the termination of the rectum is sometimes called cloaca. It stretches 

 forwards on the dorsal aspect, or to one side of the alimentary canal, 

 and may hook backwards anteriorly, or in the large Nematoda, even be 

 disposed in several longitudinal coils. It is divisible according to pecu- 

 liarities of structure, into the testis proper, which is terminal, vas deferens, 



1 Joseph states (Z. A. v. 1882, p. 605) that if the digestive tract of a living Ascaris megalocephala 

 be injected with Carmine-albumen solution, and the worm be kept at the natural degree of warmth 

 in a portion of a horse's small intestine, its body will be found on dissection to be traversed by 

 a naturally injected system of fine branching canals, devoid of proper walls, and covering the surface 

 of all the internal organs and muscle-cells. The coloured fluid passes by osmosis into the excretory 

 vessels. 



