2. STOMA 



Till' stnictuir lit' till' sliiiiia has noiuiallv Ihtii uscil :is a 

 laxiiiuiinic cliaraitcr .since the ln(;iiiiiinn of iicniatdldK.v. Its 

 wide iisr fur tin' I'lassitii-atiim <if Kiciups iif all lanks makes a 

 liiiiroiiKli ediisideratiim of its evolution neeessarv. That its 

 nioiplioldn.v has avt'-At vahu' ran scarcely lie ilonliteil li.v aiiv 

 worker in tin' liehi l)Ut the weight that may lie (jiven to its Ki'"ss 

 form seems duliioiis. Since the method of feeding and char- 

 acter of the food itself is limited liy the stoma and its arma- 

 ture, this orKiiii is prolialily more ilirectly intlnenced liy en- 

 vironment than any other. A radical clianue of feeilinj; lialiits 

 of closely related forms would reiniire chantje in stomatal mor- 

 phology if forms are to survive. Likewise it is not at all in- 

 conceivalile that (luite unrelated nemas coming to feed in the 

 same manner might eventually liecome grossly similar as re- 

 gards stomatal morphology. For the aliove reasons one must 

 consider stomatal inorjihology very closely. 



Only lun' author, fold) (llUiO. has .'ittempled to use stomatal 

 morphology as a ma.jor character. This author made the 

 "presence'' or "alisence" of a stoma ("pharyn.v. " Inn'cal 

 capsule I'tc.) the li:isis of dividing the ''Xeniates" into two 

 classes, I.;iimi;i :ind ,\I;\iinia. This classification based upon a 

 single character tot;illy withcnit correlation with an.v other 

 organ has nothing in coninum with other classifications. Though 

 their viow|ioints ma.v lie iliametricall.v oiiposeil, the classifica- 

 tions of other writers all have considerable in common and 

 they all are at eomidetp variance with that of Cobb. The 

 "presence" or "absence" of a stoma seems to be a rather 

 bad point for da.ssification purposes since all nematodes have 

 something corresponding to the stomatal region. 



Filip.iev (1934') considered enoploids such as Lrptoxo malum 

 which have no definite clear cut stoma as the more i)rimitive 

 while the writers regard forms such as Kluihililis and Phcliis 

 with distinct elong;ited stomat;i as primitive. Steiner (lO.'iS) 

 proposed a nomenclature for tin' parts of the stoma which we 

 shall follow as far as possible indicating hdmologons regions 

 and structures and the apparent evolutionary trends and varia- 

 tions as we interpret them. 



The cylindrical stoma of Shabililis may be divided into three 

 lirimary divisions (1) cheilnstom — lip cavity, (2) profostom — 

 cylindrical part of stoma and (:•!) trlontnm — end cavity, which 

 in this form is often ternu^d the glottoid apparatus. The cor- 

 responding walls of the stoma are termed cheilorhabions, pvo- 

 torlialidions and telorhabdions, respectively. The walls of the 

 protostom, i. e., protorhabdions, in Rliabdilis are less distinctly 

 subdivisible into three parts (a) prorhalidions, (b) mesorhab- 

 dions. (e) nu'tarhabdions the corresponding regions being termed 

 prostom, mesostom and metastom, respectively. In rhabditids 

 the division between niesorhabdions and metarhabdions is 

 scarcely visible in many species bnt manifests itself more 

 prominently in relateil forms. The first subdivision, i. e., pro- 

 rhabdions vs. nieso metarhabdions is qnite obvious and has long 

 been recognized as the "month collar or month cuff." The 

 basic parts of the stoma are apparently innate in the man- 

 ner of their deposition and ma.v best be observed in molting 

 specimens or in specimens treated with reagents such as weak 

 acids and alkalis. On the basis of studies made in this manner 

 it would .seem that the appearance of .pointed prntorhalidions 

 in representatives of diverse groups is not convergence in the 

 strict sense bnt rather manifestation of an innate chanicteris- 

 tic which is ordinarily masked. 



The stoma may take diverse shapes due to modifications of 

 its parts and nniny different types of arni.iture are developed 

 according to the p.'uticniar group involved. One of the most 

 common types of armann'nts are teeth. Historically the term 

 tincliio has been nuist commonly aj^plied to such structures 

 tlnnigh by derivation (oexoj = hook, the barb of an 

 arrow) its use does not seem apt. More recently the term 

 ii(l<iiili<i (65oes =: a tooth) has been apjilied and .seems the 

 more i>roin'r but in such case it should not be confused with 

 the limited definition given by Cidd) (lUlin. This author 

 defined iiilniilia as teeth arising by modification of the labial 

 region while the term uncliia was used by him to denote teeth 

 arising more posteriad. Hecanse of the common co existence of 

 both onchia and odontia, (in the Cobbian si'nse) one has need 

 for two terms .-ind the writers feel that though the first term is 

 inapt, it is lu'vertlieless worthy of preserv;ition. We there- 

 fore retain Cobb's definitions. 



Xumerous other words are commonly used in a descriptive 

 manner in specialized gronjis. Thus a region of the stoma or 

 the margin of lips may be described as ilinlah- (having teeth) 

 or ihnticiildli' (having small teeth); basal onchia in the Stron- 

 gyloidea .'ire riescribed .-is lancets; and the term foxsorex is 



used f<ir outwardly .'icting teeth at the anterior extremity 

 (oftin odontia I. In some instaiu'es, as in the tylenchids, the 

 stoma is transformed into a protrusible spear, termed a sltima- 

 liixtjil, while in others (dorylaimids ) the stoma is to a greater 

 or lessei' extent filled li.v a large tooth, in which case the spear 

 or stylet is termed an onchiostyl. Other sjiecializalions will 

 be iliscussed with the various groups. 



( '<in\'i'rgence of stomatal foi-mation accounts for the origin 

 of stylets in four separate groups: the Tylenchoidca, of the 

 rii.'ismidia, the Si|ilionolaimidae ( .Monhysteroidea ), some repre- 

 sentatives ( .tiiiiiiini)iil<x) of fhe Camacolaimidae ( I'lectoidea ) 

 and the entire l>ory]aimina. Simil:ir convergence .'iccounts for 

 liaired jaws formed essenti:illy by tlu' stomata rather than the 

 lips in the Kalic('jihalid;ie ( Strongyloiilea I , Icthyocephalidae 

 (Oxyuroidea) and Camallanidae and Cucullanidae (Camalla- 

 noidea). One must be very hesitant in concluding relationships 

 based upon such characters. 



In describing the stoma it is the common practice to speak 

 of certain jiarts or regions as being chitinized. As will be seen 

 later, there is no real evidence that either the stoma or the 

 denticular structures are actually chitin and we shall use the 

 noncommittal tei'm arlrritf i^rd for hardened i-efi'active regions. 



A. I'lIAS.MIDIA 



In each of the large groups of the I'hasmiilia some forms 

 e.xist that possess a cylindrical stonni verv similar to that of 

 Ulidbrlilix. 



HiiADDlTlNA. This suborder (Fig. rA) is divided chiefly 

 on the base of the stoma into two superfamilies, the Kliabdi- 

 toidea in which the stoma is not transfornu'il into a .stylet and 

 tlie Tylenchoidoa in whi<'h such a transformation has taken 

 place. 



In the Khabditoidea there are two families, Hh.-ibditidae and 

 Rhabdiasidae, in which the stoma is of a generalized structure 

 consisting of cheilostom, iirotostom and telostom, the iirotostom 

 being cylindrical, not surrounded by strong esophageal tissue. 

 The parts are all well sclerotized and divisions of the protostom 

 are not distinct. It is interesting that in the parasitic genera- 

 tion rhalidiasids have a relatively short ( Rlinhilias) or sub- 

 globular (Entomcla.i) stoma with well sclerotized walls show- 

 ing no indications of cheilostom or telostom. This transforma- 

 tion takes place in the development of the inilividual after 

 it enters the host. 



The family Cylindrogasteridae (Loni/ibiirca, Cjilindnipastrr) 

 is probabl.v the next most closely related group and herein we 

 again find a cylindrical stoma, distinct cheilorhabions and 

 telorhabdions (in form of small plates) and a greatly elon- 

 gated protostom subdivisible only into pro- and meso metas- 

 toms. Cephalobids, differ considerably in stomatal appearance, 

 there being a more or less cylindrical stoma in PiiiKif/nilaimus 

 and a collapsed stoma in Acrobeloidc.i and Ceplialubus. In all 

 cases the stomatorhabdions are rather separate, giving the im- 

 pression of a segmented stoma due to areas lacking in sclero- 

 tization; the extent of "degeneration" in stomatorhabdions is 

 apparently correlated with the amount of esophageal muscula- 

 ture surrounding the stoma. With complete collap.se of the 

 stoma (Duiibiii/lia) there is an entire absence of sclerotization 

 and the base of the original stomatal region (telostom) is in- 

 dicated only by a break in the esophageal musculature. The 

 consequent "stomatal region of the esophagus" is termed a 

 vrstibiilc. Such a vestibule is all that remains in the related 

 family Steinernematidae {XcoapUcta7Ui). 



The Diplog.-isteridae is a highly variable grouji containing 

 forms which link it with the Rhabditidae, Cephalobidae, Stron- 

 gyloididae and Tylenchidae. Several series of genera are 

 known in the Diidogasterinae. IthiibditiddiiU x has a cylindri- 

 cal piotostom sclerotized as in Ithnbditis but the cheilorhab- 

 dions are non sclerotizeil and the tebirhabdions .-isymnn'trically 

 developed; the closely related genus AcrnslicUii.i has <listinct 

 cheilorhalidions liut the )prostom .■ind mesostom e;ich form a 

 distinct cavity, followed by a modified met;istom containing a 

 large dor.sal tooth. Oddiildplinri/ii.r and Biitliriiix may be eon 

 sidered further nu'mbers of this "dotible stoma" series. Xrn- 

 diploffaxti r may be considered as a sirle branch of such a series 

 originating from a form not unlike Klinbdilidiiidix in which 

 collapse of the stoma was followed by loss of sclerotization of 

 stom.-itorhabdions with conseipunt convergence with cejihalobids 

 and steinernematids in the resulting vestibule. .\ second line 

 of evolution seems to be indicated by a series from Rhabiliti- 

 doidi'x to MiiiiiDirliiiidis ii mi rirniiiix, Diploi/axtrr ficlor, Prixfion- 

 rliiix aeriviira and t<'rminate with l.iicolnimux. In this series 

 there is coll.-ip.se ami non sclerotization of the metarhabdions 

 accompanied by .shortening ami thickening of the jiro- and 

 mesorhnbdions, the development of a massive dorsal and a 



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