274 NEMATHELMINTHES. 



in the free-living forms. While in the Acanthocephala mouth and 

 alimentary canal are completely absent, the Nematoda and Nemato- 

 morpha possess a mouth at the anterior end of the body, an 

 oesophagus, and a straight digestive canal, which usually opens by 

 the anus on the ventral surface near the hind end of the body. 

 The excretory organs have various forms, and their nature is not 

 understood : there are no flame-cells. In the Nematoda they consist 

 of paired canals in the ectoderm, which open by a common pore on 

 the same surface (ventral) as the anus. In the Acanthocephala there 

 is a pair of organs which appear to be of the nature of nephridia. 

 The absence of cilia may be stated as a general characteristic of the 

 group ; but they are said to be present in the supposed nephridia of 

 the Acanthocephala. 



With a few exceptions the Nemathelmintlies have separated sexes, 

 and the male organs often open into the rectum and are provided 

 with copulatory spicula. The larvae and sexual animals are not 

 unfrequently distributed in two different hosts. 



The majority of the Nemathelmintlies are parasites either during 

 the whole period of their life or at certain stages. There are, 

 however, also free -living forms which often show the closest 

 relationship to the parasitic members of the group. 



Class I. NEMATODA (THREAD-WORMS).* 



Nemathelmintlies, u-ith mouth and alimentary canal. With longi- 

 tudinal muscles only, with lateral lines, without cilia. Tlie vas deferens 

 opens into the rectum. They are principally parasites. Dioecious. 



The Xeniatodes possess an extremely elongated thread-like body, 

 which may be provided with papillae at the anterior pole in the 

 region of the mouth, or with hooks and spines within the oral 

 cavity. The mouth leads into a narrow oesophagus, which usually 

 has thick muscular walls, a chitinous lining, and a triangular lumen, 



t Besides the older writings of Rudolphi, Bremser, Cloquet, Dujardin, 

 compare Diesing, " Systcma helmintlium," 2 Bde. Wien, 1850-51. Diesing, 

 "Revision der Nematoden," Wiener Sitzungsberichtc, 42, I860. Claparede,"-De 

 la formation et de la fecondation des oeufs chez Ics vers Ncmi/todcs," Geneve, 1856. 

 A. Schneider, " Monographic der Nematoden" Berlin, 1866. R. Leuckart. 

 "Untcrsuchungcn ilbcr Trichina spiralis," Leipzig and Heidelberg, 1866, 2nd 

 edition; also "Die menschlichen Parasiten," etc., torn. 2., Leipzig and 

 Heidelberg, 1876. C. Glaus, " Ueber Leptodera appendiculata" Marburg, 1868. 

 0. Btitschli, " Untersuchungen liber die beiden Nematoden der Periplaneta 

 orientalis," Zeitzschr. filr Wiss. Zool., torn. 21., 1871. And "Beitrage zur 

 Kenntniss des Nervensystems der Xematoden," Archiv. fur Mikr. Anatomic, 

 torn. 10. A. Goette, "Unters. z. Entwick. d. IVurmcr," Leipzig, 1882. R. 

 Leuckart, "Ncue Beitrage z. Kent. d. Bau u. d. Lcbensgeschichte d. Nematoden" 

 Leipzig, 1887. A. E. Shipley, "Nemathelmintlies," Cambridge Natural History, 

 vol. 2, May, 1896. 



