CHAPTER V 

 CEPHALIC STRUCTURES AND STOMA 



Tlio various structures which go to niako up tlie copliulic 

 region of ncinatodcs cannot be classified iu a single category. 

 Yet tlieir study is naturally interlocked liotli in [iractical and 

 develoiunental anatomy, l.'nder the general hi'ading "Cephalic 

 structures" we shall discuss lips, iiseudidaliia, cephalic ivapillae, 

 cephalic setae, amphids, proludae, collarettes, cordons and 

 labial dentition while under the heading "Stoma" that part of 

 the digestive tract Ijetween the oral opening and the anterior 

 end of the esojihagus will be considered. Of necessity, a dis- 

 cussion of cei)halic structures must include parts of the ner- 

 vous system, external cuticle and sometimes stomatal develop 

 ments. Likewise, a discussion of the stoma overlaps to some 

 extent both with the cephalic structures and the esophagus. 



1. CEPHALIC STRUCTURES 



Cephalic structures have been used, to a limited extent, as 

 taxonomic characters since the appearance of Schneider's mono 

 graph (IStKi') whicli included in face, as well as lateral and 

 medial views of the anterior extremity of many of the larger 

 nematodes. Such studies were extended liy de Man (188G- 

 1907) investigating free living nenias and von Drasche (1883) 

 working with parasitic nemas of Diesing and Molin's collec- 

 tions. Certain generalities came to be accepted as a result of 

 the observations of Schneider and von Drasche. These were 

 as follows: (1) That ascarids and heterakids have three lips, 

 one dorsal and two subventral; (2) That spiruroids have two 

 lateral "lips" and (3) That parasitic nemas generally have 

 four submedian and two lateral cephalic papillae. The first 

 two of these jioinfs are for the most part acceptable to us today 

 but the third is no longer tenable. In i)arasitic nemas Looss 

 (1902) introduced the use of cephalic structures in strongylid 

 taxonomy causing them to be considered an integral part of 

 generic and specific descriptions in this group but, apparently 

 due to lack of interest or inadequacy of parasitological tech- 

 nic, little advance was made beyond Schneider, von Drasche, 

 and Looss until very recently. In free-living nemas somewhat 

 more steady progress has been made, partially attributable 

 to the smaller size which makes critical study convenient and 

 partially due to more widespread technical training. Though 

 numerous workers have contributed to our knowledge of free 

 living nenias, the chief impetus has come from the work of 

 Cobb and Steiner. The glycerin .jelly technic (see Cobb, 1920, 

 and Chitwood and Wehr, 1934) developed in the laboratory 

 under Cobb was introduced to the various visitors and asso- 

 ciates; this technic made the stuily of nemas from en face 

 practical. 



Knowledge of the liasic anatomy of the anterior end is due 

 to the contributions of Goldschmidt (1903), JIartini (1916), 

 and Hoeppli (192.'5). The confusion of two types of sensory 

 organs, tactile structures (papillae) and chenioreeeptors (am- 

 phids) in parasitic nemas caused much misunderstanding 

 though Goldschmidt recognized the difference between the dor- 

 .sal lateral organ (amphid) of Ascaris Jnmhricoidcs and the 

 other sensory organs. The same differences both in the ter- 

 minal sensilla and the internal nervous connection were brought 

 out by Zur Strassen (1904), Looss (1905), and Martini (1916) 

 in Siphonalaimus spp., Ancylostoma duodenale and Oxyuris 

 rqui, respectively. Zur Strassen even went so far as to state 

 definitely that the dorsal lateral organ of Ascaris is the same as 

 the circular lateral organ of Sipliniwlaimiis. Other -n-orkers 

 entirely ignored these observations until Cobb (1913) renamed 

 the lateral organs amphiilx defining them as paired cephalic 

 structures of specialized (unknown) function. It remained 

 for the same author later (1923, 1924, 1928) to establish the 

 general existence of pore like amphids in parasitic nemas 

 through observation and constant reiteration that they are 

 not "lateral papillae." Since then information has gradually 

 accumulated showing their universal presence in the Nema 

 toda. We shall not go into their internal anatomj- at the 

 present time since they are connected with the nervous system. 

 It will be sufficient to note that each amphid is essentially a 

 lateral or dorsolateral organ connected internally with the 

 laieroventral commissure and with a gland (Fig. 3). Near 

 the external orifice there is a dilation of the gland duct (am- 

 phidial pouch) in which nerve fibers terminate (the sensilla) 

 (Fig. 8) ; the pouch is connected with the exterior either 

 directly by an amphidial tube and pore or it opens into a 

 pocket, circle or spiral external cuticular modification. In 

 this part, only the external manifestation of the amphid (i. e., 

 pore, pocket, spiral, etc.) will be considered. 



In 191H Filipjev introduced the external form of the am- 

 phids of free-living nemas as a family and subfamily charac- 

 ter, later (1929, 1934 a, b) raising its use to suborders. 



In the meantime information regarding the number and 

 arrangement of cephalic sensory organs in both parasitic and 

 free living nemas was .■iccumulating and Chitwood and Wehr 

 (1932, 1934) brought out papers on the evolution and basic 

 jilan of cfjihalic structures with special reference to parasites 

 while Stekhoven and de Coninck (1933), de Coninck (1935) 

 and Stekhoven (1937) brought out similar papers with spe- 

 cial reference to free living nemas. Differences in findings 

 are for the most part matters of interpretation due to op- 

 posed schools of thought; the differences being in basic 

 philosophy as to the evolution of nemas and not in the organ- 

 isms themselves. The one school, represented by Filipjev, 

 Stekhoven, and de Coninck regard polymyarian nemas as 

 primitive and meromyarian nemas as neotenic while the other 

 school, represented by Looss, Steiner and the writers, consider 

 meromyarian nemas as primitive and polymyarian nemas as 

 more highly evolved. The consequences are that each group 

 sees the Xematoda from a separate point of vantage. 



The basic plan of the anterior end appears to be six lips, 

 two subdorsal, two lateral, and two subventral. On the sum- 

 mit of each lip there is a papilla, these six papillae constitut- 

 ing the internal circle and being known as internodorsals (id), 

 internolaterals (il), and internoventrals (iv) ; situated more 

 posteriorly on each of the submedial lips there are two papillae 

 while on each of the lateral lips there is one papillae; these 

 papillae constitute the external circle and have been named 

 according to their position (Fig. 8a) : dorsodorsals (dd),latero- 

 dorsals (Id), ventrolaterals (vl) (or externolaterals, el), lat- 

 eroventrals (Iv) and vcntroventrals (vv). All of the members 

 of the external circle are seldom exactly the same size or at 

 exactly the same level. Stekhoven and de Coninck (1933) 

 would therefore speak of them as constituting two circles 

 and in some forms this is indeed the ease. However, the 

 papillae of the external circle are not always segregated in 

 the same pattern. Thus the ventrolateral papillae tend to agree 

 w'ith the dorsodorsal and ventroventral papillae in their rela- 

 tive development in the Aphasmidia while they tend to agree 

 with the laterodorsal and lateroventral papillae in the Rhab- 

 ditina, Strongylina and Ascaridoidea. For that reason we 

 regard the external circle as subdivisible into two papillary 

 groups. 



Fusion and reduction of cephalic papillae commonly modify 

 the apparent cephalic arrangement but one can practically 

 always recognize remnants of the original papillae and all 

 cases may be explained in terms of the diagram presented 

 (Fig. 8A). 



As pointed out by Chitwood (1932) and the writers (1933) 

 the cephalic papillary nerves are hexaradiately ^symmetrical 

 and one would expect a hexaradiate symmetry to be basic for 

 the papillae. Therefore, the external circle should consist of 

 12 papillae instead of 10. However, no rudiments of a dor- 

 solateral pair are know-n except in some species of the Mon- 

 hysteridae and Linhomoeidae. If these forms were the more 

 primitive, one would expect to find rudiments of the afore- 

 mentioned papillae in other groups and this is not the case. 



The bilaterally symmetrical amphids are separately inner- 

 vated and cannot be considered a part of the cephalic papillary 

 symmetry. Unlike the papillary nerves, the amphidial nerves 

 enter the nerve ring indirectly, through a commissure and their 

 original position probably was posterior to the labial region 

 as indicated by embryonic rhabditids and adult aphasmidians. 

 Likewise, the amphidial orifice was probably larger and a bit 

 like the plectoid amphid, if one is to interpret on the basis 

 of embryonic rhabditids. As pointed out by the writers (1933) 

 one cannot assume any existing form to represent the pro- 

 tonematode but if one combines characters of the genera 

 Sliabditis and Plectii.i a conmion denominator of all nematodes 

 is found. One cannot interpret aphasmidians entirely in terms 

 of Ehabditis nor phasmidians in terms of Plectus, but the con- 

 verse is moderatelj' natural. Thus the amphids and papillae 

 are basically labial in i)osition in phasmiilians while the am- 

 phids in aphasmidians are basically postlabial (a more primi- 

 tive arrangement) and some of the pai)illae may be postlabial 

 in position (a less primitive arrangement). 



Kegarding the basic number of lips, there are two choices. 

 One may assume primitive triradiate symmetry in accordance 

 with the symmetry of the esojihagus as did Baylis and Daub- 

 ney (192C) or a hexaradiate symmetry in accordance with the 



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