THE NEMATODE FAMILY GNATHOSTOMIDiE. 255 



(1893) in the same species an elaborate arrangement by which 

 ducts from the two cervical sacs of each side unite into a common 

 duct, each common duct .piercing the corresponding lip and 

 opening on its outer surface by a conspicuous pore 0*013 mm. 

 in diameter. The same author (1904 a) has also described a 

 similar arrangement of ducts in Tanqua tiara. 



von Rtitz (1900), for Gnathosto'ma hispidum, and Leiper (1909), 

 for G. siamense, follow the very circumstantial description of 

 V. Lin stow. 



The ballonets, or rather their contents, seem to be refei'red to 

 by V. Linstow as the " Marksubstanz " of the head-muscles, but 

 their essential nature, as closed membranous sacs, has escaped 

 the attention of all these observers. Now, in viewing the hea,d 

 of one of these worms as a transparent object, it is easy to 

 mistake the muscles that run across the head-bulb for continua- 

 tions of the cervical sacs, or ducts connecting them with the 

 exterior. One of us (Baylis, 1916) fell into tliis error in the 

 case of Tanqua, a.nd suggested the presence of pores on the 

 anterior sui'faee of the "striated swellings" {i.e., the head-bulb). 

 This is the chief mistake into which other observers have 

 probably fallen. The "pore" on the outer surface of the lip 

 described by v. Linstow in Gnaihostoina coi-responds in position 

 to the lateral papilla which we find throughout the subfamily. 

 This papilla sometimes terminates in a little "dimple" of the 

 cuticle, which might give it the appearance of a pore ; further- 

 more, when seen in transverse section, as in v. Linstow's case, 

 the large nerve running up to the papilla might easily give the 

 impression of a duct. The central portion of the nerve then has 

 i\. reticulate appearance, and under a low power rather suggests the 

 lumen of a duct. 



We have been able to trace the cervical sacs into the ballonets, 

 but are unable to find any external opening, in series of sections 

 of Echinocephahis and of Tanqua (text-fig. 10), and thei'e seems_ 

 to be no reasonable doubt that the arrangement is similar in all 

 the genera and species. 



Subfamily Diagnosis. 



Gnathostomin^. 



Gnathostomidfe : with a cuticular head-bulb provided either 

 with mar-ked transverse striations or with rows of backwardly- 

 directed hooks, and containing four membranous, submedian 

 ballonets, the cavity of each of which is in communication with 

 one of four elongated, blind, cervical sacs hanging freely in the 

 body-cavity. 



Habitat (adult) : stom.ach or intestine (exceptionally other parts 

 of the body) of fishes, reptiles, and mammals. 



Type-genus: Gnathostoma Owen, 1836. 



