dicates the Trichuroidea developed from forms very 

 similar to pi-esent day mermitlioids. 



Baylis (1938) supplies a further discussion of the 

 host distribution of nemas. 



Embryology. In all of the nematodes thus far in- 

 vestigated the embryology may be considered identical 

 for comparative purposes. Bilateral determinate cleavage 

 must certainly have been a characteristic of the nemic 

 ancestor; the resultant adult must have been composed 

 of few large cells, probably about 600, exclusive of the 

 reproductive system. Only one to two eggs were pro- 

 duced at a time. Furthermore, a very close phylogenetic 

 relationship should be supported by similar tissue origins 

 from the primary somatic stem cells arising at the first 

 five cleavages (see Section II Charter II). 



Ontogeny. The primitivity of nieromyarian nemas has 

 been concluded (p. 219) on the basis of embryonic de- 

 velopment as well as comparative anatomy. Oligocyty 

 in the intestinal epithelium (p. 101) and cylindricity of 

 the stoma (p. 231) seem also to have been characters of 

 the primitive nema. 



Comparative Anatomy. The primitive nema will be 

 de.scribed, in so far as present evidence permits. Uni- 

 versal attributes of nemas must be included. While 

 they do not limit the ancestor, they define potentialities 

 which must have existed at the origin of the group. 

 Among the universal attributes are a layered, protein 

 cuticle formed as a deposit (differentiated) by the hypo- 

 dermis, longitudinal somatic muscles, a pseudocoelome, 

 a triradiate esophagus, and a tubular intestine com- 

 posed of one cell layer. In addition, it seems clear that 

 the hypodermis (p. 34) must have been composed of a 

 few large cells, the cell bodies grouped in four longi- 

 tudinal chords, modifications therefrom (Tn'churis) being 

 secondary. The somatic musculature is presumed to have 

 been nieromyarian (p. 40). Transverse somatic muscles 

 and specialized muscles (p. 42) might conceivably be 

 remnants of a more widespread system of muscles; 

 hence a double system of musculature is not out of the 

 question. Whether or not the coelomocytes should be 

 interpreted as a system on the wane (reduced parenchy- 

 ma) or more ancient but homologous to the parenchyma 

 system of the Platyhelmintha is not clear. We tend to 

 accept the latter view. The esophagus and esophago- 

 intestinal valve may be considered primary invagination 

 (stomodeum) while the stoma is secondary invagination 

 (hence more comparable to the ciliated "S;hlundrohr" 

 of rotatorians). Cuticular lining of esonhageal and 

 rectal structures must be presupposed in the primitive 

 nema. As to the intestine, a few large cells lined by a 

 bacillary layer (possibly reduced cilia) seems obvious. 

 Rectal glands are by no means universal but their exist- 

 ence in the majority of phasmidian groups and at least 

 in some aphasmidians (Anaplectus grcnudosiis) indicates 

 a primary existence. 



As to the nervous system, much can be said, but little 

 proved. Apparently all nemas have paired amphids con- 

 nected indirectly (through a commissure) with the nerve 

 ring; these amphids consist of a terminal pocket into 

 which sensory terminals and a gland empty. Postembry- 

 onic development (p. 229) indicates that they were post- 

 labial in position. Circumoral tactile papillae are also 

 universal and they connect directly with the nerve ring. 

 Serial sublateral tactile organs are, for the most part, 

 confined to one subclass, the Aphasmidia, but might 

 well be primitive. Specialized lateral tactile organs 

 (deirids) are confined to the other subclass (Phasmidia). 

 Posterior lateral organs, phasmid.-^, similar in structure 

 to the amphids are also confined to the Pha.~midia but 

 might well be primitive. Male tactile organs are practically 

 universal but whether the paired .system of the Phas- 

 midia or the unpaired system of the Aphasmidia is 

 primitive we cannot say. The fundamental architecture 

 of the central nervous sy.stem is so set that it leaves 

 little room for question. An anterior commissure, the 

 nerve ring, connects the dorsal, subdorsal, lateral and 

 subventral cephalic ganglia with each other and with 

 the chief nerve which is a more or less double system 

 of incompletely separated ganglia. The nerve ring is not 

 as completely closed ventrally as dorsally indicating that 

 closure may have occurred recently. From the nerve 

 ring six anterior cephalic nerves and a lateroventral 

 commissure connect with anterior Sensory organs, while 



eight direct innervation processes connect with anterior 

 muscles and six posterior motor nerves (the mediodorsal 

 and subventral as well as the laterodorsal and latero- 

 ventral somatic nerves function as motor nerves). One 

 or two lateral sensory nerves are also connected with 

 the nerve ring. The ventral or subventral nerves are the 

 chief nerves of the body and connect through commis- 

 sures with the reproductive system, rectum, copulatory 

 organs and other nerves. 



The excretory system (p. 125) has been considered as 

 of double ontogenetic origin. One could interpret the 

 one-cell aphasmidian system as most primitive but it 

 would not account for the tubular phasmidian system. 

 The terminal duct cell of the H-shaped system is ap- 

 parently the homologue of the aphasmidian system and, 

 if so, the H-shaped system should be considered primitive. 



The reproductive system is a very real stumbling 

 block. Seurat's concept of simplicity (p. 191) is easily 

 accepted but ontogeny and comparative anatomy of free- 

 living nemas entirely oppose both Steiner's and Seurat's 

 ideas concerning parallel paired gonads opening post- 

 eriorly in both sexes. Even in the male, we must for 

 the time being presume opposed gonads as primitive, at 

 least at the time nemas originated. 



Sublateral hypodermal glands and caudal glands must 

 be considered possible attributes of the nemic ancestor. 

 In conclusion, we will reemphasize the point that from 

 each organ system studied, we concluded that the primi- 

 tive nema must have been a composite of plectoid and 

 rhabditoid characters and only that with such an organism 

 could the later progeny be explained. If this is true, 

 we might expect a saprozoic life in moist soil or swampy 

 habitat. 



Relationships with Other Groups* 



The above requirements with respect to cuticle (in 

 some representatives at least), esophagus and intestine 

 are met with in the Rotatoria, Gastrotricha, Echinodera 

 and Tardigrada. In all of these groups, except the 

 Echinodera, the pharynx (i. e., muscular foregut) is 

 triradiate, a term which would be applicable to the 

 nemic esophagus; the esophagus in these forms corre- 

 sponds to the esophago-intestinal valve in nemas. 

 The musculature and body cavity conform in the first 

 three groups. There is as yet no parallel to, or explana- 

 tion of, the innervation process of the nemic muscle 

 cell which seeks the nerve rather than vice versa. This 

 apparent contradiction may not be as basic as it seems 

 superficially (see p. 172). The nervous system presents 

 some contrasting points. If we assume a "gravitational 

 hypothesis" the nemic nervous system might be ration- 

 alized. The ventral nerve is partly paired and the am- 

 phidial nerve connection would be direct were the sub- 

 ventral nerves separate and rather lateral in position. 

 If the bulk of the cephalic ganglia were similarly shifted 

 dorsad, the nemic nervous system would correspond 

 remarkably not only with that of the above mentioned 

 groups but also with that of the Platyhelmintha. Even 

 in rotatorians, which are usually characterized as having 

 a "dorsal" nervous system, the two subventral nerves are 

 the chief nerves of the body and the anterior ganglia 

 (brain) are often lobed so that they are lateral as well 

 as dorsal. A cylindroid body form and vibratile to 

 serpentine locomotion could have caused the ventral 

 gravitation of the system. The acanthocephalan and 

 nematomorphan systems could be rationalized with the 

 Rotatoria-Platyhelmintha system in the same manner. 

 There is no especial similarity of either with that of 

 the nematode. We would rather assume convergence. 

 Thii? gravitational view is the one adopted by annelid 

 and arthropod morphologists (see Snodgrass, 1935) for 

 the formation of paired ventral nerves in those groups. 

 It also helps to explain the unpaired ventral nerve of 

 echinoderids (Fig. 145) which are otherwise so much 

 like gastrotrichs. 



The excretory systems of rotatorians, gastrotrichs, 

 echinoderids, acanthocephalans and platyhelminths are 



*In the following pages the Pearse (1936) system of uniform 

 endings is followed. With minor exceptions, the comparative anatomy 

 as herein presented supports the Pearse Classification. The inclusion 

 of the Class Nemertea within the phylum Platyhelmintha is not 

 supported. It is here placed as a phylum in the series Parenchy- 

 mata. The phylum Nemata (Cobb. 1919) Pearse, 1936 is sub- 

 stantiated. 



195 



