676 THE ANIMAL KINGDOM. 



CLASS NEMATODA. 



Unisegmental Vermes, with a narrow elongated body of more or less 

 cylindrical shape and tapering to each end. There is a well-developed ciiticida, 

 derived from a subcuticula or hypodermis ; a peripharyngeal nerve-ring from 

 which six nerves pass forwards and six backzvards; a digestive tract divided 

 into three sections^ oesophagus^ mesenteron, and rectum^ and two excretory 

 tubes opening by a common anterior and ventral pore. The sexes are sepa- 

 rate; the male iisually possesses special coptdatory organs in the shape of 

 spicules and of a bursa. Ciliated epithelium is universally absent. Free- 

 living or parasitic. An Alternation of Generations occurs among some of the 

 parasitic genera. 



The body is of a very uniform shape. The anterior portion is fila- 

 mentous, the posterior thicker in the genus Trichocephalus ; so too in the 

 female Trichosoma, though to a less degree. The head is dilated in 

 Ichthyonema globiceps. Cuticular spines may be present, and folds as well, 

 extending down each side of the body, to a greater distance in the female 

 than in the male. The bursal membranes of the latter (infra, p. 681-2) must 

 be carefully distinguished from these folds. The male is usually smaller 

 than the female, and its tail is differently conformed, either strongly curved 

 at the apex, or provided with a bursa, and invariably beset with a number 

 of sensory papillae. It is sometimes forked, e. g. in Pseudalius ( Prosthe- 

 cosacter] from the lungs of the Porpoise. The free-living genera are all 

 small. Among the parasitic the female Dractmcidus (Filaria) medinensis 

 may attain a length of six feet and the female Etistrongylus gigas, which 

 inhabits the kidneys of various mammals, that of three feet or more. 

 The greatest number of parasitic Nematoda infest in the sexual state 

 the various classes of Vertebrata. 



There is a chitinoid cuticle, which is thin and delicate in the smaller 

 Nematoda, but in the larger becomes much thickened. It then consists 

 of an outer thin and more resistent layer, prolonged inwards to a greater 

 or less distance at the external apertures of the various organs, and 

 an inner thick layer, said to be subdivisible into secondary laminae. 

 The outer layer is frequently marked by transverse lines which give the 

 body a ringed appearance more or less pronounced. The inner layer is 

 fibrous, and the fibres of the different laminae decussate with one another. 

 Cuticular processes occur in the shape of circumoral fringes, of papillae or 

 spines, and lateral membrane-like expansions, variable in breadth and 

 extent. A subcuticula or hypodermis lies beneath the cuticle as a more 

 or less granular layer with sparse nuclei, most numerous posteriorly ; but 

 in larger species, such as Ascaris megalocephala from the Horse, A. lumbri- 

 coides from Man, the subcuticula and its longitudinal thickenings contain 

 numerous fibres which take a more or less circular direction, and have 



