332 PROCEEDINGS OF SECTION D. 



The dorsal and ventral ridges are only slightly developed, but 

 are very definite. The four small remaining intervals are occupied 

 by the muscle cells, the protoplasmic portions of which are com- 

 paratively small. As one passes backwards this well-defined and 

 symmetrical arrangement rapidly becomes lost with the increase in 

 diameter of the worm, the great thinning out of the wall and con- 

 sequent increase in the size of the body cavity resulting in the 

 marked asymmetry described above. 



Nervous System.— [See figs. 5, 6, 7, 32-34, 36, and 37.) This 

 together with the excretory system is very difficult to observe, and 

 what one does observe is difficult to interpret. The nerve ring, 

 which is usually indistinct, especially in the male, lies from 0.172 

 to 0.188 mm. in the male and 0.142 to 0.188 mm. in the female from 

 the anterior end. These variations have no relation to any condi- 

 tion of possible contraction of the worm. The most common dis- 

 tance is about 0.170 mm., that given by Cleland and Johnston, 

 namely, 0.18 mm,, being less frequent. 



So far as can be determined at present the nerve bands appear 

 to form an irregular plexus around the head, in which what are 

 possibly nerve cells are to be seen. No anal ganglia have been 

 detected. Behind the nerve ring, with its nerve cells, the nerve 

 fibres are arranged as a tiny bundle in the centre of the dorsal and 

 ventral median bands, and in a more or less diffuse manner in the 

 inner parts of the lateral bands {see fig. 37). With the change of 

 structure of the longitudinal bands, as we pass backwards along the 

 body, the character of the longitudinal nerves alters also. 



Dorsal and ventral nerve strands are present, consisting of very 

 few fibres, six to eight, which lie in the ordinary hypodermis of the 

 body wall, no swollen or projecting bands being present. 



The lateral nerve elements are, however, much more developed, 

 though even here often impossible to find with certainty. In 

 general it may be stated that there are two lateral longitudinal 

 nerves more or less well developed on each side, as shown in figs. 

 33a, b, c, 34, and 36. In the sub-cuticular layer of this region there 

 are also sometimes to be found, even when the lateral nerves are 

 difficult to determine, a row of nuclei {compare fig. 33a) which are 

 apparently connected, at all events at intervals, by fibres with the 

 lateral nerve strands. At other times these nerve fibres from the 

 lateral nerves run out very definitely and end in branches imme- 

 diately under the cuticle, though no sense papillae have been detected. 

 These nuclei may extend over the whole lateral band or may only 

 show over its dorsal and ventral two-thirds. 



Excretory System.— {See figs. 6, 7, 32, 33, 34, 35, 36.) This 

 when best defined consists in main part of the body length of a 

 double or single canal {see figs. 33a and 33b) which lies between 

 the two lateral nerve strands in the lateral band. Like these nerves, 

 the excretory canals are sometimes hardly distinguishable on one 

 side of the body {compare fig. 32), while in the same section the 

 jateral band of the other side may be so vesicular in character 



