CHAPTER VI 

 THE ESOPHAGUS INCLUDING THE ESOPHAGO-INTESTINAL VALVE* 



1. GENERAL MORPHOLOGY 



The esopliagi of nematodes are extremely diveree in both 

 gross anatomy and mode of function ; this diversity is corre- 

 lated with the widely differing feeding habits as well as with 

 the phytogeny of this organ, the result being an assembly of 

 numerous types with examples of convergence and of diver- 

 gence. In addition features of no apjiarent present value to 

 the organism are often preserved. 



The esophagi of all nematodes have a few characteristics in 

 common, for example their fundamental triradiateness, one ray 

 of the lumen being directed ventrally (though torsion may 

 cause a complete reversal in some regions), a cuticular lining; 

 marginal nuclei and fibers connected with the rays of the lumen 

 or railii, radial muscles and nuclei located in the regions be- 

 tween the esophageal radii; nerve cells of the esophago-sym- 

 pathetic system; and glands emptying into the lumen of the 

 esophagus or into the stoma. 



The esophagus in cross section has three sectors (Fig. 6'i) , 

 a dorsal sector and two subventral sectors. Cell walls are not 

 distinguishable in the esophagus as a whole; it is, therefore, 

 syncytial. However, the cell walls of nerve cells are usually 

 distinct and the protoplasm of each of the esophageal glands 

 usually retains its identity. The nuclei of the marginal and 

 radial fibers are so placed as to leave no doubt that each be- 

 longs to a .specific part of the fillers and therefore functions 

 separately. 



Through their contraction the radial muscles cause dilation 

 of the esophageal lumen; the marginal fibers are apparentl.v 

 static rather than contractile and the marginal tissue and 

 protoplasm, according to Martini (1908), correspond to the 

 epithelium and form the cuticular lining of the esophagus. 

 The esophagus is often covered externally b.y a semicuticular 

 membrane which may possibl.v be a product of the marginal 

 tissue. Longitudinal supporting fibers are usually absent in 

 the esophageal wall. The esophageal glands apparently serve 

 a digestive capacity, the secretion being either ejected through 

 the mouth in case of extracorporeal digestion or passed into 

 the intestine with the food and aiding in intracorporeal diges- 

 tion. Other possible functions of the esophageal glands will 

 be discussed later. 



Nemic esophagi evidence man.v degrees of cell or nuclear 

 constancy; Looss (1896) first discovered nuclear constancy 

 with reference to the radial, marginal and gland nuclei in 

 ascarids. Since this observation, representatives of the major 

 groups have been studied liy us with the result that we find 

 many large groujis characterized by a certain number of nu- 

 clei of a given type, arranged in a definite manner ; loss of 

 nuclear constancy is characteristic of certain groups, and evo- 

 lution from forms with few cells or nuclei to forms with many 

 cells or nuclei nia.v be traced. 



Other points of systematic value in the stud.v of nemic esoph- 

 agi are: The form of the lumen and esopliageal lining, the 

 position of the esophageal gland orifices, the form of the 

 gland ducts and tubules, and finally, the form of the esophago- 

 intestinal valve. 



The esophageal lumen itself may be triradiate, the sides of 

 the radii converging distally (Fig. 65B), or they may terminate 

 in incomplete tubes (Fig. (ifiA). In either case the esophageal 

 lining may have thickened attachment points at which the ra- 

 dial muscles are inserted (Fig. (i.5B) ; whenever the radii ter- 

 minate in marginal tubes, or whenever attachment points are 

 l)resent, the radial muscles are said to be concentered (Fig. 

 6.5C) while in other instances thev are usuallv dispersed 

 (Fig. 6.-.D). 



The dorsal esophageal gland orifice is usually at or near the 

 anterior end of the esophagus. In some cases it is fituated on 

 a dorsal tooth projected into the stoma (Strongylus, Ancylos- 

 tanid, OticlidlaiiniiK) hut in other cases the orifice is far re- 

 moved from the anterior end (Aphrlrnchaiilrs Prionchulus, 

 Diirjihiiniiis) . The subventral glands, on the contrary, usually 

 have their orifices in the posterior part of the esophagus (meta- 



corpus) and only in rare instances do they extend to the an- 

 terior end (Enoploidea, Tripyloidea, Dioctophymatoidea). As 

 a general rule, the duct of an esophageal gland is situated in 

 the middle of a sector and branches are given off into the 

 protoplasm from each side of the central duct. In large forms 

 it has been found that these branches terminate blindly in the 

 form of tubules and the protoplasm is not in direct relation 

 with the lumen. Onl.v the terminal duct is lined with cuticle. 

 Because of the numerous cases of convergence in gross form, 

 the structural diversity will be discussed group by group unit- 

 ing >)oth the gross anatomy and the histology in order that 

 evolutionary trends ma.v be more readil.v observed. 



A. SUBCLASS PHASMIUIA 



All phasmidians have one point in common with respect to 

 the esophagus, namely, the subventral esophageal glands always 

 have orifices far removed from the anterior extremity. On the 

 basis of gross morphology they show no obvious separation into 

 groups but on the basis of histology and developmental anatomy 

 they are divisible into two groups, the orders Rhabditida and 

 Spirurida. In the Rhabditida the esophagus shows evidences, 

 in its larval stages at least, of being composed of two major 

 parts, an elongate corpus and a short bulbar region, while in 

 the Spirurida it is composed of a short muscular anterior part 

 and a long wide glandular part. These are fundamental dif- 

 ferences established in the histology of the organ. 



Rh.-vbditin.A-. The esophagus of members of the sub- 

 order Rhabditina consists of a cylindroid corpus (further divisi- 

 ble into procorpus and metacorpus*), a narrow isthmus and a 

 pyriform or elongate bulbar region. In some forms, such as 

 Mhahditis (Fig. (i.l), the bulbar region contains a well de- 

 veloped valve, while in others, such as DipJut/aster (Fig. 76), 

 no such valve is present. It will suffice for our purposes to 

 describe the esophagus of Ithabilitis and compare the other 

 forms with it. 



The esophageal lumen of Rliabditix ti-rricula varies accord- 

 ing to the region, the rays of the lumen of the procorpus ter- 

 minating in well developed marginal "tubes" (Fig. 67ab), 

 those of the met.-icorpus in smaller "tubes" and those of the 

 isthmus and bulbar region in acute angles. The lumen is 

 modified at the valve and the lining thickened (Fig. 68c). The 

 procorpus contains IS nuclei: six radial nuclei (ri-„) in one srt 

 (RI) near the base of the procorpus, one being on each side 

 of each sector (Fig. 67a-b), and 12 nerve cell nuclei (ni-n) ar- 

 ranged in three chains, one in the center of each sector. The 

 radial muscles are concentered in six liands corresponding to 

 the six radial nuclei. The metacorpus (Fig. 66 & 67c-d) con- 

 tains 28 nuclei: three bilobed marginal nuclei (nii-j) in one set 

 (MI)t, six radial nuclei (r7-i=) in one set (RID, and 19 nerve 

 cell nuclei (ni.i-,,i) forming three chains as in the procorpus. 

 The isthmus is anucleate while the bulb contains 30 nuclei, as 

 follows: six marginal (nu-n, nh-a'*, three of which are in the 

 prevalvar region (Mil) and three in the postvalvar region 

 (Mill); 12 radial (r,:,-,s, r,„-j,, r-i.-;.,), a set of six of which 

 are in the prevalvar region (RIII) and groups of three in 

 the valvar ( R IVa) and postvalvar regions (R IVb) ; nine 

 nerve cell nuclei (uii-.n) and three esophageal gland nuclei 

 (gi-a). The esophago-intestinal valve has a simple triradiate 

 lumen ; its wall consists of an internal layer of transverse 

 fibrous tissue containing two n\udei and an external circular 

 la.ver containing three nuclei. 



The esophagus of Hhabflilis functions as follows: Contrac- 

 tion of the radial muscles in the procorpus tends to triangulate 

 the lumen thereby increasing its volume; thereafter contrac- 

 tion of the radial muscles of the metacori>us cause dilation of 

 its luiiH'n while relaxati(m of the mu.scles of the ])rocorpus and 

 dilation of the bulb;ir valve occur. In order to discuss the 

 nu'chanism of the bulbar valve we shall label the jiarts; the 

 esophageal lining has a .series of three thickened regions. Dur- 

 ing rest the first piece (1) is convex anteriorly while the 



*StrictIy speaking (tie esopliHKo-intcstiiml valve i.s part of the csdplia- 

 gus. Howevpr. in conformity with roniinnn usage, the term esophagus 

 a.s usHd herein excludes the esophago-intestinal valve unless otherwise 

 specified. 



*These terms are suhstituted for jirecorjuis and postcorpus (p. 10) 

 at the suggestion of Dr. Steiner. 



tXot uncommnnly flip marginal nlU'lei may aj>pear to lie double, in 

 forms with marginal tubes at the termination of esophageal radii; in 

 such cases the lobes are designated mi„.n)i,,, etc. 



76 



