right subventral tooth and finally by complete amalgamation 

 of these structures in Lycolaimus. The third and last line of 

 evolution seems to antedate Ehabililoides in that the cheilor- 

 habdions are preserved. Ti/lopJiari/n.r and Tylcnchudon have a 

 stoma that clearly simulates a stomastostyl, the stylet guide 

 being formed by the cheilorhabdions, the anterior part of the 

 stylet by prorhabdions, the shaft (or stylet) by meso-metar- 

 habdions and basal knobs by telorhabdions. Such forms are 

 retained in the Diplogasteridae since there is no proof that 

 the "stylets" are protrusible. TylopJiarynx am\ Tylenchodon 

 form a definite link with the Tylcnehidae. We have omitted 

 remarking thus far on the presence of sclerotized rugae in the 

 Diplogasterinae. As has been previously noted, members of this 

 group have either no lips or much redneed lips. In many forms 

 the prostom is longitudinallj- broken into numerous heavily 

 sclerotized rugae, these rugae having much the same appear- 

 ance as the internal leaf crown of strongylids. The degree of 

 development of the rugae seems to be correlated with the size 

 and degree of development of th dorsal tooth {Pristionchiis 

 aerivora, Diploi/dxter fictor, Mononchoiiles iimeric(inus). As to 

 the origin and end of the series previously mentioned, there are 

 some who would see the series in reverse, i. e., proceeding from 

 short stoma, amalgamated forms such as Lycolaimus to cylindri- 

 cal stoma forms such as BJiahditiiloides. In reply we may state 

 that study of young larvae show the series to be correctly 

 oriented. Forms with a short or divided stoma in the adult 

 stage have a more cylindrical, elongated, less divided stoma in 

 the larval stage. 



It has been previously noted that the Tylenchoidea (Fig. 55) 

 have a stomatostyl homologous with the stoma of rhabditoids, 

 probably having developed through some such forms as Tylen- 

 chodon or Tylophdrynx. Such a stylet consists of four ))asic 

 sclerotized parts (l) cheilorhabdions (st.vlet guide), (2) pro- 

 rhabdions (conoid or insertable part of stylet), (3) meso- 

 metarhabdions (stylet shaft) and (4) telorhabdions (stylet 

 knobs). The stylet guide or cheilorhabdions are best devel- 

 oped in tylenchids with a sclerotized cephalic region such as 

 Enploloiiniis and Pratyloichiis but may be relatively distinct 

 in less heavily sclerotized forms such as Xeotylcjichus. The 

 basal knobs (telorhabdions) are primarily three in number but 

 may be bilobed, particularly in cases where they are anteriorly 

 bent such as Boplolaimiis bradys* In tylenchids the greatest 

 diversity is in degree of development of the basal knobs and 

 relative size of the stylet. A few forms such as Apheh nchus 

 avcnae, have no basal knobs and a very delicate stylet. Others 

 such as AvJiclrnclidides parieliniis have weak knobs and a deli- 

 cate stylet while Ditylenclnis dipsaci has both moderately de- 

 veloped. Hoplolaiinu.'! bradys represents a type with massive 

 stylet while Paratylcnchus and Criconema have a massive and 

 greatly elongated stylet which may extend posteriorly into the 

 metacorpus. A few instances are known in which there is sexual 

 dimorphism in the degree of stylet development. 



The entire family AUantonematidae is characterized by stylet 

 degeneration, the cheilorhabdions and telorhabdions being most 

 effected by this tendencj". Stylet development in this famil}- 

 seems to be primarily dependent upon the life history, the stylet 



Fig. 59. 



Cephalic regions in the Filarioidea. 

 A, E-F, J-L— Dipetalonematidae. B, 

 G-I — Filariidae. C — Desmidocercidae, 

 D — Stephanofilariidae. X — Dirofilaria 

 immitis; B — Dieheilonema rheae ; C — 

 Desriiidocerca ttuniulicfi ; D — Stephano- 

 filfiria stilffii; E — Elaeopkora schnfi- 

 deri; F — Dipetnlon^ma gracile ; G — 

 Squnmofilaria thoracis; H — Setaria 

 critiltw; I — Diplotriaena sp. : J — C(ir- 

 dUtnftnn ristlidinis ; K — Litomosoides 

 haitilitti: L — Litomosa amer'ictnia. E — 

 .After Wehr & Dikmans, 1935, 7.ooI. 

 Anz. V. 110(7-8). G — After Tubangui, 

 1934, Philipp. J. Sc. v. 55(2). L — 

 After Mcintosh & Mcintosh, 1935, Proc. 

 Helm. Soc. Wash. v. 2(1). 



Returning to the other subfamily of the Diplogasteridae, the 

 AUoionematinae we find less variation in stomatal develop- 

 ment. In AUoionema and E)iabdHophancs the cheilostom and 

 prostom form the functional stoma and their rhabdions are 

 well sclerotized but not always distinctly recognizable; the 

 meso-metarhabdions may or may not be well sclerotized but 

 they are always surrounded b,y esophageal tissue and the telor- 

 habdions are rudimentary. Strone/yloides ra7isomi (Strongy- 

 loididae) passes through this stage in its larval development 

 preceding complete collapse of the niesostom with vestibule for- 

 mation SclcnrcUa of the AUoionematinae apparently proceeds 

 even further in this line of evolution with shortening and amal- 

 gamation of the eheilostom-prostom, the result being a nearly 

 complete convergence with Lycolaimus* 



The remaining two families of the Ehabditoide.a are insuffi- 

 ciently known for a general characterization of their stomata ; 

 we cite the figures of Aiii/iostoiiia ph lliodonlis (.\ngiostomati- 

 dae), Vicelis nira and Vnr/rlla .s(ctn (Driloneniatidae) as repre- 

 sentatives of the groups. 



*The Rliabditinae — AUoionematinae — Diplogasterinae complex seems 

 to exhaust practically all of the possible combinations of stoniatal, 

 esopliageal and bursal cliaracters. Aside from ttmth formation — absence 

 of valved bull) and ceplialic cbara(;ters tliere is little or no correlation. 

 One might easil.v prefer to arrange the narrow bursate cylindroid stoma, 

 forms of the Rhabditidae (Rlifihtlifnidr.s. RhiibditrUa) and Diplogas- 

 terinae (lihtthditidoides. Nfridipliif/iiyfrr) together an(J the short stoma 

 Lj/coUiimUH with SehnepIIa in the AUoionematinae. However, on tlie 

 basis of present evidence this would cause an unwarranted confusion 

 and would he even more arbitrary than the present division. 



being best developed in very young larvae and degenerating or 

 entirely disappearing after whatever stage has passed in which 

 the organism enters its host animal. Thus Cobb (1928) showed 

 that the stylet is vestigial in adult males while it is well de- 

 veloped in non-gravid females; the males and females copulate 

 while free-living after which the male dies but the female re- 

 enters its host. The stylet of the female also becomes de- 

 generate after entrance into the host. 



Stronoylina. Stomata in the suborder Strongylina 

 (Fig. ri(i) give one of the clearest cases exemplifying the bio- 

 f/cnetic law that ontogeny recapitulates phylogeny. Though 

 the structure in the adult stage is highly varied in the first 

 stage larva it is always of a more or less rhabditoid form. In 

 adult strongyloids the stomatorhabdions are always heavily 

 sclerotized, the stoma well developed and capacious while in 

 adult trichostrongyloids and metastrongyloids the stomato- 

 rhabdions are usually weakly sclerotized and the stom,a re- 

 duced or rudimentary. Though the stoma becomes reduced or 

 rudimentary in many representatives of the Strongylina, such 

 reduction occurs through shortening rather than through over- 

 growth of esophageal tissue. Vestibule formation which oe- 



*Tho anterior part of tlie stylet, as well as the cheilorhabdions are 

 cast off with the exuvium at the last molt of Diti/trnrhus. It likewise 

 diiTers in chemical nature. However, the extent of molting is no evi- 

 dence of hfunology for the entire stomatal and esophageal lining is 

 cast off at tile last molt of CtntiaUatuts, Anct/lostoma and Af/amprTnis 

 while apparently only the cheilorhabdions molt in Rhabditii) and first 

 .1 nri/lftxtitmti larvae. 



66 



