second and tliird pieces (2-3) nearly touch one another. Con- 

 traction causes a reversal of the position of 1, point a (Fig. 

 97E-F) becoming nearly a.xial and point 6 moving from an 

 axial position to a point formerly occupied by o. This is ac- 

 complished by contraction of the radial muscles of the pre- 

 valvar region (associated with ris-is) and possibly of the 

 valvar region (associated with rw-si). Opposition to this move- 

 ment, i. e., return to the position of rest, is accomplished by 

 muscles of the valvar region (rni-2i). Movement of piece 1 

 to a position of dilation is followed by dilation of the lumen 

 opposite pieces •'-■:} in series (through contraction of the post- 

 valvar radial muscles associated with r-<«-2i), opening of the 

 esophago-intestinal valve and, finall.v, closure of this structure. 

 There is no evidence that the marginal fibers are ever con- 

 tractile. They appear to function entirely in the capacity of 

 "fixed points" upon which the sectors are "hinged." In the 

 corpus, relaxation proceeds slowly while contraction or dila- 

 tion is spasmodic. Perhaps the tubular form of the esophageal 

 radii contributes to the opposition of the radial muscles be- 

 cause of their elasticity. Muscle fibers definitely opposed by 

 other muscle fibers are found only in the valvar region of the 

 bulb. All muscle fibers of the esophagus are perpendicular or 

 oblique to the esophageal axis; there are no circular or longi- 

 tudinal fibers. 



In Rhabditis. the dorsal esophageal gland orifice is at the 

 anterior end of the procorpus (Fig. 76), the short cutieularly 

 lined terminal duct is followed by a small ampulla and a long 

 canal extending to the bulb where it becomes lost in the mass 

 of dorsal esophageal gland protoplasm; the canal extends near- 

 ly to the base of the bulbar region where the gland nucleus is 

 situated. The subventral gland orifices are situated at the 

 base of the metacorpus and, likewise, each is provided with an 

 ampulla and a canal leading jiosteriorly to the bulbar region. 

 The mass of the subventral gland protoplasm is lateral in 

 position (i. e., in the lateral part of the subventral sectors) 

 and their nuclei are in the valvar region. 



The nerve cells, previously mentioned, form the esophngo 

 sympathetic system which consists of a nerve trunk in the 

 center of each esophageal sector, these trunks being connected 

 by tliree commissures, one at the base of the corpus, one in 

 the prevalvar region and one in the postvalvar region. This 

 system is connected with the central nervous system by means 

 of a pair of nerves from the subventral trunks through the ex- 

 ternal surface of the procorpus. 



Other members of the superfamily Rhabditoidea with a 

 valved bulb apparently have the same structure as that de- 

 scribed above. In T)ipl<ic/as1ir (Fig. 76), and similar forms, in 

 which the valve is absent but the radial muscles of the Vnilbar 

 region not degenerate, slightly larger marginal "tubes" occur 

 in the corpus and the radial muscles associated with rio-n seem 

 to act together instead of as two separate groups. Sliabdia.i 

 and Strviigyloides present peculiarities in that the esophagus 

 has a well developed valve in the free-living generation and 

 first stage larva of the parasitic generation, but this structure 

 degenerates in the later development of the parasitic genera- 

 tion though there is no change in the number or arrange- 

 ment of the esophageal nuclei. 



In the superfamily Tylenchoidea, actual degeneration of the 

 radial muscles of the bulbar region takes place but the .30 

 nuclei of this region which correspond to those in Rhahfliti.i 

 still remain (Figs. 70 71). This degeneration of the muscula- 

 ture is correlated with increase in size of the esophageal glands 

 which form practically the entire bulbar region in forms such 

 as Uitylcnchvs Jipxaci and may even extend beyond the ba.se 

 of the esophagus as one or more eso])hageal aiipendages as in 

 Aphelenchim avenae (Figs. 7.5-76). In a few forms, such as 

 Aphcleiirliiix and Aplirlewhoides, Cobb (1923) showed that the 

 dorsal gland orifice is situated in the anterior part of the 

 metacorjius instead of the anterior i)art of the procorpus as 

 is generall.y the case. In many tylenehids the metacorpus is 

 unusually large and acts as the chief "pump" of the esopha- 

 gus, while in the Allantonematidae the musculature of the 

 entire esophagus appears degenerate. 



Stronoymna. — Members of the suborder Strongylina have 

 !in esophagus which is grossly clavate to cylindroid but in 

 sonu' instances f.'iint indications of procorpus, metacorpus. 

 and bulbar region are observable in the adult. The first stage 

 larvae of strongyloids and many trichostrongyloids have an 

 esophagus identical with that of Hhabdilis : during the second 

 and tliird stages the esojihagus becomes more elongate and 

 the valves of the bulbar region disappear resulting in an 

 esophagus reminiscent of that of Diplofia.iirr except that the 

 metacorjius is not enlarged or as distinctly set off. With pro- 

 gressive development the various regions become grossly oblit- 

 erated to a greater or lesser degree. Metastrongyloids <liffer 

 from strongyloids aiul most trichostrongyloids in that first 



stage larva does not have a valved bulb, but more closeI.r 

 corresponds in its morphology to the second and third stage 

 larvae of the other two superfamilies. 



The detailed structure of the esophagi of strongylins was 

 studied by Jagerskiold (1897), Looss (1905), Imminck (1921, 

 1924) and the writers (1934, 1935). It was found that the 

 marginal, radial, and gland nuclei agree in number with those 

 of Eltabditis but the prevalvar radial nuclei (r,,i-io) are ar- 

 ranged in two groups of three nuclei each, near the center of 

 the sectors. There is no essential difference in arrangement of 

 the nerve cells though there is considerable variation as to the 

 total number observed (from 29 in iletastrongyliis dongalns 

 to 44 in Strongylus eflentatum). The triradiate esophago-in- 

 testinal valve seems to be composed of seven cells. The sub- 

 ventral esophageal gland nuclei are quite minute in members 

 of the Strongyloidea and thus f.ar gland orifices near the 

 nerve ring have been observed but rarely. This may be corre 

 lated with the hypertrophy of the dorsal gland characteristic 

 in such forms, and it may have a distinct bearing on the feed- 

 ing habits. 



The musculature is always "concentered" rather than "dis- 

 persed" in strongylins, and the lumen may have marginal 

 tubes as in Mctasfronpyhis or there may be a series of thicken- 

 ings of the esophageal lining forming attachment points as in 

 (h'xiipjiiigo.stoiiiiim deiitatiim (Fig. 7S) and Ancylostoina duo 

 denale. 



ASCARIDINA. — More work has been done on the esophagi 

 of representatives of the suborder Ascaridina than of repre- 

 sentatives of other groups. It was in Parascaris equoriim that 

 the orifices of esophageal glands were observed for the first 

 time in any nematode by A. Schneider (1866). Subsequent 

 work has been carried out bv Jagerskiold (1893, 1894), Ham- 

 ann (1895), Looss (1890), Ehlers (1899), Jerke (1901), K. C. 

 Schneider (1902), Goldschmidt (1904, 1909, 1910), Martini 

 (1916, 1922), Kulmatycki (1918, 1922), Allgen (1921), Muel- 

 ler (1929, 1931), Hsii (1929, 1933), Plenk (1924, 1925, 1926), 

 Chitwood (1931), Chitwood ,^nd Hill (1931), Chitwood and 

 Chifwood (1933, 1934, 1936), de Bruvn (1934), and Maekin 

 (1936). 



In ascaridins the esophagus varies in gross morphology 

 more than in any other group. The members of the super- 

 family Oxyuroidea are less diverse in detailed anatomy than 

 are the members of the snperfamily Ascaridoidea. In the 

 former group, the esophagus in the adult stage is basically 

 rhabditoid, having, with a few questionable exceptions, a cylin- 

 droid corpus, a more or less distinct isthmus, and a valved 

 bulb. In numerous forms, such as Syphacia and 0.ryuris, the 

 corjius may be clavate while in a few, such as Ransotnncnia, it 

 may be fusiform. In some forms the metacorpus is enlarged 

 and either cylindroid as in Leidyncma or ovoid as in Hammer- 

 .iclnniilliclla : in oild types the entire corpus is pyriform (Aoni- 

 riis) and in still others the procorpus only is fusiform (Ozolai- 

 miix). The isthmus also is diverse in gross appearance, some- 

 times being greatly elongated as in Atractis or Aorurus, some- 

 times only moderate in size as in Tlielastoma, sometimes 

 recognizalile only as a groove as in Ehigonema and some- 

 times subglobular as in Labidiiriis. Less diversity occurs 

 in regard to the bulb; it is valved except in a few genera 

 such as Drrmatoxys and Lciperenia but the degree of develop- 

 ment of the valves differs greatl3' between the typical rhabdi- 

 toid form of Thelastoiim and the much reduced type of O.rytiris. 



One would ])resume that forms such as Lridynrtiia (Fig. 70) 

 and HdinmcrxchmidticUa represent the primitive t.vpe since 

 the metacorpus is enlarged as in Rhahditis and since careful 

 study proves the corpus to be subdivided into pro- and meta- 

 corpus even in forms in which these parts otherwise are not 

 grossly separable. However, primitivity apparently does not 

 appl.y here since in the genera Leidyncma and Hammerschmid' 

 tirlla the esoiihagus does not reach this stage of development 

 until adulthood of the female. The .juvenile females, as well 

 as the adult neotenic males, have a c.vlindrical corpus as in 

 Thelastoma. Sexual dimorphism in the esopliagus as well as 

 cephalic structures, stomata, and cuticular ornamentation is 

 one of the outstanding oddities of the Thelastomatidae and 

 Ransomnematinae. Developmental modifications, without aji- 

 parent bearing on relafionshi]), have also been described in 

 O.ryiiris iipii by Ihle and Van Oordt (1921) and Wetzel 

 (1930); in this species the fourth stage larva has a pseudos- 

 toin fornu'd by the dilation of the entire corpus (Fig. 97 O). 



In their detailed anatomy, the oxyuroids show less diversity. 

 The lumen of the corpus u.sually shows the marginal "tubes" 

 as in Rhabditis and sometimes in addition (or sometimes in- 

 stead of them) the esophageal lining has distinct cuticular 

 thickenings or attachment points for the radial muscles. The 

 three esophageal glands (some statements to the contrary) 

 open as in IHnihdilis, i. e., the dorsal m-ar the base of the 



78 



