84 MORPHOLOGY OF INVERTEBRATE TYPES 



interstitial lamella at the end of which is the excretory canal. 

 The entire canal is nothing but a long cylindrical cavity inside 

 of a single enormous cell extending almost from end to end of 

 the worm. The canal bends downward a little behind the 

 nervous ring and unites with the excretory canal of the other 

 side forming a short single duct. We have seen already that 

 this duct opens ventrally in the median line and that this ex- 

 cretory opening is situated about two millimeters from the an- 

 terior end. The wall of the common duct is also formed by 

 a single cell. Accompanying the excretory canals throughout 

 their length in the side lines are two pairs of excretory glands, one 

 pair in each side line. In a cross-section they appear as large 

 ovals situated in the main tissue of the line, one above, the other 

 below the interstitial lamella. 



Attached to the inner edge of the side lines not far from their 

 anterior end are two pairs of phagocytic organs which have the 

 appearance of small tufts. Their function is to retain and to 

 make substances harmless which cannot be removed by the 

 excretory canals. 



Nervous system. The nervous system of Ascaris is com- 

 posed of several cephalic ganglia or nerve centres situated in the 

 region of the oesophagus, a nerve ring surrounding the latter, 

 eight longitudinal nerves, six anterior sensory nerves and an 

 anal ganglion or nerve centre in the posterior end of the body. 

 The largest and best developed of the anterior ganglia is the 

 so-called ventral cephalic ganglion. It has two lobes and is con- 

 nected with the other ganglia and the rest of the nervous system 

 through the nerve ring which represents a large cephalic com- 

 missure. Connected with the nervous ring -and running for- 

 wards are six bundles of sensory nerves supplying the sense 

 organs of the lips. Posteriorly the nerve ring gives off six 

 longitudinal motor nerves. One of these is ventral and is the 

 heaviest of all; another is dorsal, two are subdorsal and two 

 sub-ventral. The dorsal nerve is inclosed in the dorsal median 

 line, the ventral in the ventral median line. The other four 



