482 



FRESH-WATER BIOLOGY 



KEY TO NORTH AMERICAN FRESH-WATER NEMATODA 



1 (64) Intestine normal and functional throughout; anus present in both 



sexes 2 



The forms which are included here are typical nematodes. They possess an alimentary 

 canal which is complete and functional during the entire life of the individual. They are 

 free living in the adult as well as in the larval stage of existence. With the free-living forms 

 are sometimes found parasitic forms so similar in structure that a knowledge of their source is 

 needed to determine whether the species is parasitic or not. No note is taken of the parasitic 

 forms and the following statements apply only to the true free-living nematodes. 



They are all relatively small in size and so transparent that the internal structure can be made 

 out clearly in the Uving animal. In these respects as well as in detail of internal structure they 

 stand in distinct contrast to the other group included under the alternative heading in the 

 key. Famihes which include only parasitic species are not mentioned in this key. 



2 (13) Oral end armed with roptrusile spear or sting 3 



3 (8) Spear with bulbous base 4 



4 (s) Cuticula with 70 to 100 coarse, retrorse annules Iota Cobb. 



Genus consisting of a considerable number of species, found in swamps and in acid 

 soils. These nematodes are covered with retrorse scales, or bristles, so that it is practically im- 

 possible for them to move in any other direction than forward. ^ Near the head the remarkably 

 large and powerful spear can be seen through the skin. When, in order to make punctures, this 

 spear is thrust out, the nematode is not pushed backward, because of the friction which its 

 scales offer to surrounding soil particles. But often the males of Iota lose the spear at the last 

 moult and become relatively longer and more slender and smoother, and then they look very 

 unlike the females. 



Representative species Iota octangular e Cobb 19 14. 



^ 14 21 .__2i_ -85" 92^ ^^ Male unknown. Habitat: Dismal Swamp, Va. 



C"^ II 12 12./ 9 6. '""'■ 



Fig. 775. Iota ocfangulare. 

 a, mouth opening; b, lip region; c, spear muscles; d, shaft of spear; e, base of spear; /, cuticular tube 

 ol esophagus; g, nerve-ring; k, posterior portion of esophagus; i, flexure in ovary; j, body muscles; 

 k: cuticula; I, one of the eight longitudinal rows of modified cuticula; m, ovum; n, muscles of body wall; 

 0, sublateral modification of the cuticula; p, uterus; q, subdorsal modification of the cuticula; f, vulva; 

 s, muscles of the body wall; t, rectum; u, anus; v, terminus. (After Cobb.) 



5 (4) Cuticula with 200 or more finer or almost invisible annules 6 



ing amphids. ^ Oblique line, conventionalized drawing of the outlet of the excretory- 

 duct, placed just behind the measurements relating to the nerve-ring. ^ Character 

 of the hne running through the formula (see adjacent table) , and dots placed on either 

 side of the Hne. ^ Absence of short horizontal Hnes above and below main line, such 

 marks being used when wings are present. ^ Horizontal stroke under two-thirds of 

 the nerve-ring \vidth measurement. ^ A corresponding stroke under three-fourths of 

 the width measurement for the base of the neck. ^^ Single quotation marks around the 

 measurement indicating the position of the vulva, and 71 used as an exponent. ^^ Ab- 

 sence of spinneret mark, — an angular sign used to indicate spinneret. ^ Dash in 

 front of the M and 63 used as an exponent. ^^ Curved marking under the transverse 

 anal measurement, extending to the end of the formula line of the male, ^^ 4 and 5 

 used as sub-figures before and after the anal diametral measurement with ditto marks 

 to indicate that the ribs occur on both sides. 



