BY N. A. COBB. 177 



Excretory System. 



I have reserved this system until the last because I have the 

 most to say concerning it. It consists simply of a one-celled 

 gland emptying ventrally opposite the posterior bulb of the 

 oesophagus by means of a short and narrow duct, (Fig. 2, v.) 

 Its form strongly reminds one of the ventral excretory organ as 

 it exists in a large number of free-living Nematode genera. 

 Its position, however, alters rapidly with the growth of the 

 animal, becoming situated in the very first stages of larval growth 

 quite behind the oesophagus. Its later history I do not know from 

 observations on O x y u r i s v e r m i c u 1 a r i s, but in Oxyuris 

 o b V e 1 a t a it is as follows : It exists at first in the form here 

 shown for vermicular is. It rapidly increases in size pos- 

 teriorly and becomes bifurcated. Each of the branches moves 

 to the adjacent lateral region and becomes attached to the lateral 

 field thereof, continuing all the while to increase posteriorly. 

 (Figs. 3, 4, 5.) It thus gives rise to the two lateral ducts which 

 are so often described in Nematodes as uniting and emptying 

 ventrally through the porus excretorms. 



It seems to me that much confusion with regard to the excre- 

 tory system of Nematodes has arisen in a very natural way. The 

 oft-mentioned lateral ducts or lateral vessels were first discovered 

 in some of the larger parasitic forms (Ascaris lumbri- 

 coides, &c.), and were afterwards found to exist in many 

 parasitic species, being sometimes imbedded in the sub- 

 stance of the lateral fields. The nature of these vessels was 

 at first uncertain. They were held by some naturalists to consti- 

 tute a circulatory system, but by others were regarded as part 

 of a water- vascular system. The fact that these vessels almost 

 universally emptied through a single ventral opening of small 

 dimensions gradually led to the opinion, solely on morphological 

 grounds however, that the two vessels were excretory in their 

 function. The small ventral pore was given the name of porus 

 excretorius. The connection with the lateral fields led however 



to a very natural morphological error, namely the opinion that 

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