Characteristics of Nematelmintlies. cxxxvii 



entoparasitie life in the human subject, which is so favourable in 

 this creature to the reproduction of the species. The tenacity of 

 life, as against desiccation^ which has been supposed to characterize 

 many members of the class^ is possessed in reality by only a few 

 land and fresh-water genera, Tj/lenchus, Cephalohus, Aphelenchus, 

 and Plectus ; and in them is to be considered as partly, but, as 

 it would appear, not wholly, dependent upon the power which they 

 have of maintaining their tissues in a moist condition, and which 

 they owe to the absence from their integument of the pores so 

 characteristic of other free Nematoids. 



The Chaetognatha, an order of mai'ine worms of small size, represented 

 by the single genus Sagitta, have been here ranked with the Nematoidea 

 and Acanthocephali as Nematelminthes, instead of being placed in a 

 separate Class. The differences which separate them from the free 

 Nematoids appear to be in no respect of more than ordinal importance, 

 consisting mainly in the facts of their hermaphroditism, and of their 

 possession of transversely-striped muscles. The peculiar armature of the 

 mouth, with two laterally and two dorsally-placed series of setae in the 

 Sagittae, is obviously homologous with the intra-oral armature so common 

 in Nematoidea ; and the production of the external layers of the integu- 

 ment into fins, supposed to be characteristic of the Sagittae, is often 

 observable both in the free and in the parasitic representatives of the 

 order with which we are comparing them. The same remarks apply to 

 the cuticular setiform spicula, and to the muscular structin-es of the 

 Sagittae. The interior of the digestive tract in Sagitta is lined with 

 ciliated epithelium, and Claparede has described a species, Sagitta Cephor- 

 loptera, which has a semilunar area in its nuchal region surrounded with 

 a band of cilia. Thus the Sagittae, and through them the entire class of 

 Nematelminthes, come to coincide with the rest of the Sub-kingdom 

 Vermes in the possession of ciliated epithelium. 



The animal described by Claparede, Anatomic und Entwickelungs- 

 geschichte Wirbelloser Thiere, p. 88, Taf. xviii.. Figs. 2 and 3, 

 under the name of Chaetosoma opidocephalum , would appear to 

 stand as a transitional form midway between Nematoidea 

 and Chaetognatha ; whilst Gordms, and possibly also EcMno- 

 deres, described by Claparede, /. c, p. 92, and by Greef, Archiv. 

 fiir Naturgeschichte, 1869, connect the former of these two 

 orders with the somewhat aberrant Acanthocephali. 



