304 Dr. L. (Jrlej on Nematodes. 



Ehabditlclie and E-habditoid larvfe we have become acquainted 

 with transitional forms which apj^ear to connect the free- 

 living and parasitic groups. The chief of these are Rhahditis 

 irigrovenosa and appendicidatn^ which show a close connexion 

 with the parasitic genus Oxyuris.^ both from a biological and 

 an anatomical point of view. So thoroughly does Biitschli* 

 recognize this relationship that he considers it desirable to 

 unite the genera Oxyuris^ Cei^halohas^ Anrjuillala^ and Rhah- 

 ditis into one genus. I have proposed t the name Rhabditi- 

 formre for the group. Our task, then, is to discover whether 

 this group can be maintained in its independence, and whether 

 the parasitic and free-living forms are to be separated, or 

 whether a continuous chain of modifications connects all 

 Nematodes, admitting of no lines of demarcation. We have 

 thus come to the point at which Bastian commenced fifteen 

 years ago ; and it is to be regretted that the characters which 

 he put forward as distinguishing the two groups from one 

 another have not been remembered as well as they deserved. 



I shall show, in the first place, that characters exist by 

 wliich the so-called Rhabditiformai may be separated from the 

 real free-living species or Anguillulidre. 



De Man I was the first to demonstrate that our terrestrial 

 and freshwater forms have been developed from three or more 

 marine original forms ; and I have assigned to that which 

 gave rise to the Rhabilitiformaj the name of ^^ ProtoncJioIaimusJ^ 

 From this the species of Mononchus and Diplogaster are 

 developed. Diplogaster is even provided with a doubly- 

 bulbed oesophagus, a sign that it inclines to a parasitic life ; 

 but the organs usually coincident with a free existence, such 

 as the circumoral bristles and the lateral circular markings, 

 are not wanting. From Diplogaster sprang other forms in 

 two directions 5 one division has kept the bristles and lateral 

 circular markings and acquired a caudal sucker as well {Plectus^ 

 Bst.), while others have entirely lost these organs, necessary 

 to a free life ; this may be taken as a sign that they have 

 passed into a parasitic state of existence. It is just these 

 latter species which represent the group of the Rhabditiformaj. 

 All other free-living species known at present have at least 

 one of the organs I have named ; and many have eyes besides. 

 Points of ditterence occur in the mode of reproduction and 

 manner of life. 



* " Ueber freilebeiide Nematodeii,'' Zeitschr. f. wiss. Zool. Bd. xxvi. 



t Monograpliie der AnguilluHden : Budapest, 1880. (Editio separata e 

 " Termeszetrajzi fiizetek,' vol. iv. partes i., ii. 1880. A musajo uationali 

 hungarico edita.) 



X ' Oudeizoekiugen over vrij in de Aarde levende Nematoden,' Leide, 

 1875. 



