276 



NEMATH ELMINTHES. 



i.e., sheddings of the cuticular layer, seem only to occur in the young 

 forms. The muscles are each composed of a single cell, in which 

 two parts are distinguishable, a clear, sometimes a granular proto- 

 plasmic portion (medullary substance), which projects into the 

 body-cavity and is often prolonged into processes; and an external 

 fibrillated layer (Fig. 227). 



The ISTematodes may be distinguished as Meromyaria or Polij- 

 myaria, according to the arrangement of their muscular system. 

 In the Meromyaria the number of muscle-cells (which are arranged 

 according to definite laws) in the cross section is 

 small (eight), while in the Polymyaria their number 

 is considerable. In the latter the muscle-cells arc; 

 often connected together by transverse processes of 

 the medullary substance, which unite on the so-called 

 median lines to form a longitudinal cord. 



In almost every case two lateral regions remain 

 free from muscles, and form the so-called lateral 

 lines or regions, which may equal in breadth the 

 neighbouring muscular regions. These lateral regions 

 are internal projections of the hypodermis, and are 

 formed of a finely granular nucleated substance, and 

 enclose a clear vessel containing granules. This 

 vessel is connected with that of the opposite side 

 in the anterior part of the body, and the two open 

 by a common transverse slit, the excretory pore, on 

 the ventral surface in the median line. The lateral 

 lines are regarded as the excretory organs* Median 

 lines (dorsal and ventral), accessory median lines 

 (sub-median lines), the latter being placed between 

 the principal median line and the lateral line, are 

 also to be distinguished. Cutaneous glands, in the form of uni- 

 cellular glands, have been observed principally in the region of the 

 oesophagus and in the tail. 



The nervous system, owing to the difficulty which its investigation 

 offers, has only been satisfactorily recognized in a few forms. It 

 consists of a nerve ring (Fig. 228) surrounding the oesophagus, near 

 the anterior end of the body, in Asc. megalocephala just in front of 



FIG. 227. -Muscle- 

 cell of a Nema- 

 tode. 



* Hamann (Sitz. Ber. Akad. Berlin, 1891, p. 57) asserts that in Lecanocephalus, 

 in which the right canal only is present, the excretory canal is coiled, and ends 

 posteriorly in a small opening into the body-cavity. This statement needs 

 confirmation. 



