346 NEilATHELMIXTHES. 



In almost every case, with the exception of Gordius, two lateral 

 regions remain free from muscle and form the so-called lateral lines 

 or regions, which may equal in breadth the neighbouring muscular 

 regions. These lateral regions 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 

 vascular pore, on the ventral surface in the median line. The 

 lateral lines have the value, both as regards position and structure, 

 of 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 dis- 

 tinguished. The so-called ventral cord of Gordius, which may be 

 compared to the median line and has perhaps the significance of an 

 elastic rod, is very large. Cutaneous glands, in the form of unicel- 

 lular 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 recognised in a few forms. It con- 

 sists of a nerve ring surrounding the oesophagus, and sending off 

 posteriorly two and anteriorly six nerve trunks (Ascaris mcyalo- 

 cephaki). The posterior trunks run in the dorsal and ventral lines 

 (N. dorsalis, ventralis), to the extremity of the tail ; while of the 

 six anterior nerves, two run in the lateral lines (N. later ales), four 

 in the interspaces between the lateral and median lines (N. siib- 

 mediani), and supply the papilla? around the mouth. The ganglion 

 cells lie partly near, in front of and behind the nerve ring, partly 

 on the fibrous cords themselves, and are arranged in groups which 

 can be distinguished as ventral, dorsal, and lateral ganglia. There 

 are in addition groups of ganglion cells in the median lines and in 

 the lateral lines in the caudal region. ' 



As sense organs we must mention the eyes found in the free- 

 living Nematoda, and the papillae and tactile hairs found principally 

 in the neighbourhood of the mouth. Each papilla is supplied by 

 one nerve fibre, which is swollen to a knob and forms the axis of 

 the papilla. 



[The Nemato'la possess a body cavity, but are without any trace of a vas- 

 cular system.] 



Generative organs. The Kematodes are dioecious (with ex- 

 ception of the hermaphodrite Pelodyles, and of the Rhdbdonema 



