Phylum 4. Nemathelminthes {Uomd-worms). 



Tlie two groups of this pliylum are so very different from one 

 another, that it is a matter of great doubt whether they should be 

 put together. Common to both groups are the elongate, cylindrical 

 body, and the muscular body-wall which encloses a well-defined 

 body-cavity. Cilia are absent. The sexes are always separate. 



It is as yet impossible to say to wtioli of the other phyla the Round- worms are 

 most nearly allied. Prom what is known, up to the present, of their anatomy, 

 they seem to present no characteristic affinities with any other groups. 



Class 1. Nematoda {Thread-worim). 



The smooth body is almost always elongate and cylindrical, often 

 filiform, and usually pointed at both ends. It is covered with a light, 



thick, elastic, cuticle, beneath 

 r which is a thin hypodermis, and 



under this is a single layer of 

 muscle cells, of different shapes, 

 and often very large. The mus- 

 culature is, however, interrupted by 

 four so-called longitudinal areee, 

 ridges of the hypodermis, which 

 traverse the length of the body, one 

 in the mid-dorsal, one in the mid- 

 ventral line, and one on each side, 

 dividing the muscular sheath into four 

 longitudinal bands. Of the 

 four longitudinal arese, the lateral are 

 the best developed. The mouth is at 

 the front end of the body ; sometimes 

 a distinct buccal cavity lined with a 

 stiff cuticle is present, but in most forms, this is not the case. The 

 rest of the digestive tract, which takes a straight course through the 



Fig. 119. Transverse section of a 

 Nematode, diagrammatic, b ventral, 

 i gut, e excretory organ, 'h skin, g 

 gonad, m musole-oell, r dorsal area, s 

 lateral area. — Orig. 



