THE ANATOMY OF ONGHOLAIMUS VULGARIS, BAST. 143 



with branches ramifying throngh the protoplasm. The nucleus 

 is highly elongated, in places band-like, and its structure 

 very strongly resembles that of A. decipiens and allied 

 forms. 



We should expect in the embryo of a parasitic form to find 

 a transition between the type found in the free-living forms 

 and that in the adult parasite. iVs I have shown, our expecta- 

 tions are fulfilled. 



A step further can be taken in pointing out homologies; it 

 appears probable that the excretory organ of Nematodes, in 

 whatever form it occurs, is a nephridium homologous with the 

 uephridia of, e. g., Platyhelmia or Clieetopods. 



A nephridium is defined by Ray Lankester (11) as follows : 

 " Nephridia are distinguished by their independent origin, 

 each from a single superficially placed cell, which often is 

 seen to be derived from ectoderm, and probably must be 

 traced to that layer even when it appears as part of the meso- 

 blast. They are also distinguished by their structure, which is 

 primarily that of a number of perforated or drain-pipe cells 

 placed, as it were, end. to end." 



The excretory organ of Nematodes is a cell perforated by 

 an intra-cellular canal. That it consists of a single cell and 

 not of a number placed end to end does not militate against 

 the homology suggested, since the chains of cells originate 

 from a single cell. 



Janimes (7) and Jagerskiold (9) agree that it is ectodermal 

 in origin. The condition as found in the ventral gland of 

 free-living forms by many observers, and as described for 

 Oncholaimus vulgaris by myself, certainly suggests an 

 ectodermal origin. 



Hamann (5) is the only modern authority who regards it 

 as mesodermal. 



We are accustomed to think of nephridia as paired organs, 

 but a single organ arising from the midline is as much 

 bilateral as two arising one on each side of the midline. 



If the excretory organ when specialised resembles a 

 nephridium, in its most simple form, it bears a strong 



