VI 



PHYLUM NEMATHELMINTHES 



305 



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 Grordius, one of the Nemato- 

 morpha ; 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 does not exceed O5 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 Nematomorpha it consists in part of a true epithelium a 

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

 layer, which in many genera 

 has the same structure as in 

 Ascaris, i.e. consists of a single 

 layer of longitudinal fibres, in- 

 terrupted at the dorsal, ventral, 

 and lateral lines, each fibre 

 being spindle-shaped and pro- 

 duced into a protoplasmic pro- 

 cess which projects into the 

 body-cavity. But in many forms 

 (e.g., Strongylus)thQ muscle-cells 

 are flat rhomboidal plates (Fig. 

 243), and each quadrant con- 

 tains only two rows, the total 

 number in a transverse section 

 being therefore eight. In the 



Nematomorpha the muscles are interrupted along the ventral line 

 only, the dorsal and lateral lines being absent (Fig. 245). More- 

 over 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. 244, C), by means of which the worms draw blood from the 



VOL. i X 



FIG. 243. The body-wall of a platymyariaii 

 Nematode, spread out. lat. I. lateral lines. 

 (After Leuckart.) 



