THE NEMATODE FAMILY GNATHOSTOMID^. 



255 



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

 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 Tanqiia tiara. 



von Riitz (1900), for Gnathostoma liispidnin, and Leiper (1909), 

 for G. siamense, follow the very circumstantial description of 

 V. Lin stow. 



The balionets, or rather their contents, seem to be refei-red to 

 by V. Ijinstow 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 head 

 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 this error in the 

 case of Tanqua, and suggested the presence of pores on the 

 anterior surface of the "striated swellings" {i.e., the head-bulb). 

 This is the chief mistake into wliich otlier o])servers have 

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

 described by v. Linstow in Gnathostoma corresponds 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 

 a reticulate appearance, and under a low power rather suggests the 

 lumen of a duct. 



We have been able to tra,ce the cervical sacs into the Ijallonets, 

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

 of Ech.inocephahis and of Tanqua (text-fig. 10), and there seems 

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

 the genera and species." 



Subfamily Diagnosis. 



Gnathostomin^. 



Gnathostomida^ : with a cuticular head-bulb provided either 

 with marked transverse striations or with rows of backwardly- 

 directed hooks, and containing four membranous, submedian 

 balionets, 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) : stomach or intestine (exceptionally other parts 

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

 Type-genus: Gnathostoma Owen, 1836. 



