VI 



PHYLUM NEMATHELMINTHES 



283 



3. GENERAL ORGANISATION. 



External Characters. The Nematoda vary much in size : the 

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

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

 the Guinea-worm (Dracunculus) 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 Gordiacea ; 

 it is found in earth or water and resembles a tangle of brown 

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

 the diameter cloes not exceed 0'5 mm. 



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

 by the deric epithelium or external ectoderm : the latter usually 

 takes the form of a protoplasmic layer with scattered nuclei, but 

 in the Gordiacea it consists in part of a true epithelium a single 

 layer of distinct cells. Beneath the ectoderm is a muscular layer, 

 which in man}' genera has the same structure as in Ascaris, i.e. 

 consists of a single layer of longitudinal fibres, interrupted at the 

 dorsal, ventral, and lateral lines, 

 each fibre being spindle-shaped 

 and produced into a proto- 

 plasmic process which projects 

 into the body-cavity. But in 

 many forms (e.g. Strongylus) the 

 muscle-cells are fiat rhomboidal 

 plates (Fig. 228), and each quad- 

 rant contains only two rows, the 

 total number in a transverse 

 st-ction being therefore eight. 

 In the Gordiacea the muscles 

 are interrupted along the ventral 

 line only, the dorsal and lateral 

 lines being absent. (Fig. 230.) 

 Moreover the muscular layer 

 in this order is lined by a layer of epithelial cells which bounds 

 the body-cavity. 



Enteric Canal. The mouth is frequently armed with spines 

 (Fig. 229, C), by means of which the worms draw blood from the 

 intestinal blood-vessels of their host. Many free-living forms have 

 a sharp stylet for piercing the tissues of the plants on which they 

 feed, and a suctorial apparatus for absorbing their juices. The 

 posterior end of the pharynx is often dilated to form a globular 

 chamber with muscular walls, the gizzard (Fig. 231,#~.). The only 



, . 



i 





Fir:. -22S. The body-wall of a platymyarian 



Xemutiltj. spread uut. Int. I. lateral lines. 

 (After Leuckart.) 



