changes may take place in the intervening period we do 

 not know, but in other members of the group there is 

 some information. Steiner (1924) described a larval 

 ascarid (Agamascaris odontocephala) in which lips are 

 absent, the cephalic papillae well developed, and in addi- 

 tion there is a mediodorsal tooth, or agamodontium (Fig. 

 15(5 W). A similar tooth has been commonly observed in 

 larval ascaridids of fish (Anacanthocheilus rotundatus 

 (Wiilker, 1930). Presumably this tooth aids in the 

 migration of such larvae through tissues and is shed at 

 the last moult. 



Spiruroids may pass through stages possibly indicating 

 phylogenetic development but certain cenogenetic features 

 sometimes occur. Thus in the first stage larvae of Gongy- 

 lonema, Ascarops and Physocephalus, Alicata has shown 

 that there is a peculiar group of ventral cephalic hooks 

 (Fig. 157 C) and in the third stage larvae there are six 

 indistinct rudiments of lips and all except the ventrolateral 

 cephalic papillae are well developed. Between the two 

 dorsodorsal and the two ventroventral lips there is a 

 pair of median cuticular elevations (Fig. 157 K). Corre- 

 sponding structures are present in the adult stage of 

 Simondsia (Fig. 58 R). However, all spirurids do not 

 have the hooks and cuticular elevations. In Physaloptera 

 (Alicata 1937) the third stage larva has been found to 

 have papillae and lips approximately as in the adult 

 stage (Fig. 58, p. 63) ; unfortunately the first stage 

 larva has not been studied. The larva of Habronema 

 (Hsii and Chow, 1938) has six lips instead of the adult 

 two. The first stage larva of Camallanus has an hexagonal 

 oral opening and the labial region continues to be of larval 

 form until the last moult at which two "lateral jaws" 

 appear. 



Little is known of the development of labial structures 

 in the Aphasmidia. Crossman (1932) found that the 

 larvae of Tyloccphalus have a head resembling Plectus 

 in that there are four large setose papillae and during 

 development membranes or "cushions" form a web between 

 these structures. 



Changes occurring during development of the stoma 

 are often very marked in specialized nemas though little 

 change takes place in the generalized forms. In rhabditids 

 the only noticeable change is in the diameter of the 

 stoma which becomes wider with age; the absolute length 

 may not change in postembryonic development. Only 

 the cheilorhabdions and prorhabdions are cast at moult- 

 ing. Related nemas such as diplogasterids may show 

 some changes during development of the stoma. As a 

 rule, nematodes having short and proportionately wide 

 stoma in the adult stage have a much more narrow stoma 

 tending to be cylindrical or prismoidal in the larval stage 

 (Fig. 158 V-X). Thus in the development of strongyloids 

 and trichostrongyloids the stoma in the first stage larvae 

 is rhabditiform while in the second stage larvae it 

 collapses and the cheilorhabdions and protorhabdions may 

 simulate a stylet in the third stage (Fig. 158 H). In the 

 Strongyloidea there is a rather extensive reformation of 

 stomata between the fourth and adult stages. 



The stoma of the fourth stage larva is usually rather 

 short and wide and is termed a provisional buccal capsule. 

 In the late fourth stage a cavity is formed around this 



Fig. 158. 



Postembryonic development of members of the Rhabditina and 

 Strongylina. A-D — Trichostrongylus axei larvae tTrichostrongylidae] 

 (A — First stage; B — Early second stage; C — Late second stage; 

 D — Third stage). E-H — Ancyclostoma caninum [ Ancylostomatidae] 

 (E — Third stage larva, excretory apparatus; F-G-H — Head. 

 F — dorsal view and G-H — lateral view). I-L — Oaigeria 

 pachyscelis f Ancylostomatidae] Head. (I — Ventral view, 

 fourth stage ; J — Lateral view, fourth stage ; K ■ — 

 Late fourth stage; L — Moulting specimen). M-0 — Cylicostomum sp. 

 [Strongylidae] head of larva, lateral view (M — Fourth stage; N — 

 Late fourth stage; O — During fourth moult). P-S — Strongylus 

 vulgaris [Strongylidae], fourth stage larval female anterior end. 

 (P-R — Stages in formation of buccal capsule; S — Moulting speci- 

 men). T — Metastroni/ylus elongatus [Metastrongylidae] (Posterior 

 end of larva in second moult). U — Ornithostrongylus quadriradiatus 

 [Trichostrongylidae] third stage larva. (Lateral view of tail). 

 V-X — Pristionchus sp., [Diplogasteridae], stomatal region (V-? 

 first stage larva, lateral view ; W — Same specimen, dorsal view ; 

 X — Adult). Y-Z — Rhabditis strongyloides [Rhabditidae] (Y — Embryo 

 in egg shell; Z — Stomatal region, first stage larva). E-H, after 

 Stekhoven. 1927. Proc. Roy. Acad. Amsterdam., v. 30. I-L, after 

 Ortlepp. 1937. Onderstepoort J. Vet. Sc, v. 8 (1). M-O. after Ihle 

 and Oordt. 1923, Ann. Trop. Med. & Parasit. v. 17 (1). P-S. after 

 Ihle and Oordt, 1924, Koninklijke Akad. Wetensch. Amsterdam, 

 v. 27 (3-4). T, after Schwartz and Alicata. 1931, J. Parasit. v. 

 28. U, after Cuvillier, 1937, U. S. D. A. Tech. Bull. 569. Remainder 

 original. 



structure. Looss (1897) observed two such cavities in 

 Ancylostoma one dorsal and one ventral, which gradually 

 fused. In Strongylus (Fig. 158 P-S) and Cylicostomum 

 (Fig. 158 M-O) 'ihle and van Oordt (1923, 1924) observed 

 a single anterior cavity (a-c) completely surrounding the 

 provisional buccal capsule. At the base of this a septum 

 (s) is formed separating the anterior cavity and pro- 

 visional buccal capsule from the remainder of the body. 

 Behind the septum a new cavity is formed outside the 

 anterior end of the esophagus. Around it the adult buccal 

 capsule forms (Fig. 158 N & R). The esophagus then is 

 withdrawn and becomes attached to the base of the 

 adult buccal capsule (Fig. 158 O & S). The old lining of 

 the esophagus is attached to the provisional buccal cap- 

 sule. In other strongylins the stoma may remain cylin- 

 drical to the adult stage (Cylindro pharynx, Fig. 56 C). 

 .Metastrongyloids differ from the foregoing in that the 

 stoma is never rhabditiform so far as is known; mesor- 

 habdions and telorhabdions are degenerate in the first 

 stage. In the later development of such forms the stoma 

 may disappear (Metastrongylus elongatus). Young the- 

 lastomatids have a longer, more cylindrical stoma than 

 adults and the same may be said of ransomnematids while 

 in oxvurids remarkable changes have been described. 

 Ihle and van Oordt (1921) found that the larvae of 

 Oxyuris equi have a massive pseudostom (Fig. 97) formed 

 by a dilation of the corpus the dilation being entirely 

 absent in the adult. One would judge this to be a purely 

 cenogenetic feature related to feeding habits. Larval 

 subulurids have approximately the same type of stoma 

 as do the adults while the larval ascarids, like the adults, 

 have none. In the Spiruroidea Chitwood and Wehr (1934) 

 found that the stoma in some forms appears to go 

 through stages which are known to occur in the adult 

 of other forms. Thus in the case of Physocephalus six 

 cuticular projections of the prostom in the third stage 

 larvae appear to form the lips of the adult stage while 

 they retain their original larval position through devel- 

 opment to the adult stage in Ascarops. It has also be-en 

 found that the stoma is more cylindrical in the third 

 stage larva than in the adult. Ransom (1913) describes 

 the mouth cavity of the first stage larvae of Habronema 

 as shallow becoming longer and cylindrical by the third 

 stage. Passing now to the Filarioidea we find that in 

 some forms (Dirofilaria immitis) the third stage larva 

 has a well developed cylindrical stoma while the adult 

 has no distinct stoma. However, in the related genus 

 Litomosoides the stoma in the adult stage is in practically 

 the same form as it is in the larvae of Dirofilaria. Re- 

 garding stomatal changes in the Aphasmidia a little is 

 known only in the cases of mermithids, trichuroids and 

 dicctophymatoids. Christie (1936) has found that the 

 stoma of the embryo of Agamermis is represented by two 

 small plates posterior to which there is a long narrow 

 cuticular tube surrounded by esophageal tissue. Within 

 the esophageal tissue a large onchiostyl develops and 

 gradually comes to surround and replace a part of the 

 stoma or esophageal lumen at moulting. In trichuroids, 

 Fuelleborn (1923) described similar onchiostyls as de- 

 veloping in late embryonic stages and Lukasiak (1930) 

 described a stylet in larval dioctophymatoids which had 

 been removed from the egg shell. 



Esophagus. Postembryonic changes in the esophagus 

 of nemas are limited to parasitic forms, some changes 

 occurring in nearly all the large parasitic groups. Pre- 

 sumably the changes are correlated with the development 

 of new feeding habits. In general the earlier stages of 

 the esophagus are more similar to that structure in 

 Rhabditis than is the esophagus of the adult, but very 

 little or no change takes place in the number of cells 

 during development except in those nemas with re- 

 duplicate esophageal glands (see p. 233). At the time 

 of hatching thelastomatids usually have an esophagus 

 consisting of a cylindrical corpus, isthmus and valvulated 

 bulb but in a few genera of the Thelastomatidae (Ham- 

 merschmidtiella, Leidynema, Aorurus) the metacorpus 

 becomes enlarged in the adult female. Peculiarly, no such 

 change in form takes place in the development of the 

 males. Because of the late appearance of the swelling it 

 is not considered a homologue of the swelling present in 

 the rhabditoid esophagus. Oxyurids usually have an es- 

 ophagus like that of the adult during all stages of 

 development (exception Oxyuris equi see p. 78). _ No 

 particular developmental changes have (been noted in the 

 esophagus of heterakids but ascaridids present many 



231 



