vi PHYLUM NEMATHELMINTHES 299 



ORDER 2. NEMATOMORPHA. 



Nematoda in which the sub-cuticle consists of a layer of distinct 

 cells and the pharyngeal nerve-ring sends off a single large ventral 

 nerve-cord composed of three parallel strands well supplied with 

 nerve-cells. There are no excretory canals. The reproductive 

 products are discharged into the body-cavity. This order includes 

 a small number of greatly elongated, thread-like worms (species of 

 the genus Gordius), which are parasitic in the asexual, free-living 

 in the sexual stage ; and is usually regarded as comprising also the 

 genus Nectonema, which has only been found swimming in the sea. 



Systematic Position of the Example. 



Ascaris lumbricoides is one of many species of the genus Ascaris, 

 and belongs to the family Ascaridce of the order Nematoidea. 



The absence of an epithelial lining to the body-cavity, and the 

 presence of elongated gonads continuous with their ducts, indicate 

 its position as one of the Nematoidea. Among the numerous 

 families constituting this order, the Ascaridae are distinguished by 

 the possession of three lips furnished with papillae, and by the 

 body of the male being curved ventrally and being provided with 

 penial setae. Ascaris is distinguished from the other genera of the 

 family by the absence of a bulb-like enlargement at the posterior 

 end of the pharynx, by the posterior extremity of the body having 

 the form of a short blunt cone, and by the presence of two penial 

 setae in the male. 



3. GENERAL ORGANISATION. 



External Characters. The Nematoda vary much in size : 

 the little Anguillula, one of the commonest of aquatic animals, does 

 not exceed 1 mm. in length, while the dreaded parasite known as 

 the Guinea-worm (Filaria medinensis) is sometimes as much as 

 2 metres (6 feet) long. The length is always great in proportion 

 to the diameter, and the body is always bluntly pointed at the 

 anterior end and either pointed or forked posteriorly. One of the 

 most striking cases of disproportion between length and breadth is 

 exhibited by the free, sexual form of Gordius, one of the Nemato- 

 morpha ; it is found hi earth or water and resembles a tangle of 

 brown string, the length being frequently as much as 15 or 16 cm. 

 while the diameter does not exceed 0'5 mm. 



Body-wall. The body is always covered by a cuticle secreted 

 by the deric epithelium or external ectoderm (sub-cuticle). The 

 cuticle is usually smooth, sometimes beset with spines or hooks, 

 sometimes ringed. In certain species it is raised up to form a pair 

 of lateral fin-like folds running along the sides of the body for some 

 distance. Nectonema differs from all the rest in having two rows 



