As yet there seems to be too little information about 

 such races among either parasites of animals or plants 

 for any conclusions to be drawn. 



Steiner (1917) discussed the habitat distribution of 

 free-living nemas, pointing out that there are two chief 

 faunas, one terrestrial and the other marine. The fresh 

 water fauna i.s more closely related to the terrestrial 

 than to the marine and interchange is not particularly 

 common, there being no typical brackish water fauna. 

 Following the suggestion of Simroth (1891) he con- 

 cluded that terrestrial nemas are more primitive than 

 marine. This view also presumes that other early forms 

 of life (i. e.. Bacteria, Algae, Rotatorians, etc.) were 

 originally tsrrestrial or fresh water and subsequently 

 marine. 



morphologic considerations, one might argue that the 

 Anisakinae ( Ascarididae) are the most primitive since 

 they occur in fish. However, secondary entrance of 

 Anisakinae into fish which came up rivers to feed would 

 be ample explanation of their parasitism. Absence of 

 any aquatic (marine) nema with lateral excretory canals 

 makes fresh water or more typically land origin manda- 

 tory. We presume the Ascaridoidea arose from the 

 Oxyuroidea, hence f)-om inse:t parasites. These, in turn, 

 probably originated from a rhabditoid with an H-shaped 

 excietory system similar to that of Rhabditis doliclmra. 



The origin of the Spirurida is indeed dubious and 

 possibly lost in antiquity. No adult forms are found 

 in insects, millepeds or other invertebrates. The Cam- 

 allanoidea are characteristically parasites of fish and 



A— ■jl'islii'-i bniiihenae 



Wuettrrrria hanefofti 

 Acaiithocheihiicna pfrttana 



Fig. 143. 

 Geographic distribution of Necator ameriraviis and Ancylostonia 

 ihioiteniile. From Craig & Faust. Clinical Parasitology, Lea & 

 Febigor. 



Pig. 144. 

 Geographic distribution of Wnvhereria bancrojti and Dipetalo- 

 nemu perstans [Acanthocheiloneina p.). From Craig & Faust, Clin- 

 ical Parasitology, Lea & Febiger. 



Host groups. Plant parasitic nemas are usually so 

 non-specific as to host that the information derived there- 

 from is of questionable significance. Animal parasites 

 present much more usable information. Thus we note 

 that the Strongylina make their first appearance in 

 amphibians and reptiles and are found in birds and 

 mammals but not in fish. All strongylins of marine 

 mammals belong to the Metastrongyloidea. Further- 

 more the Strongyloidea and Trichostrongyloidea (both 

 without intermediate hosts) are parasites of all groups 

 above fish while the Metastrongyloidea are confined to 

 birds and mammals. On that basis one would say that 

 without doubt the Strongylina originated with the am- 

 phibians or reptiles and that the adaptation to inter- 

 mediate hosts was secondary. We would presume the 

 Strongylina to have arisen with the Amphibia and prob- 

 ably from a rhabditoid with two subventral excretory 

 glands similar to those of Rhabditis strong yloides. 



In the Ascaridina, the Oxyuroidea include parasites 

 of insects, millepeds, amphibians, reptiles and mammals 

 with a few aberrant species (Rondonia rondoni, Monhys- 

 teridcs piscicola) inhabiting fresh water fish. These 

 latter infections seem to have occurred secondarily. The 

 Oxyuroidea seem to have originated with the millepeds, 

 scarabeids and cockroaches. It is particularly inter- 

 esting to note apparently identical species of nemic 

 parasites in millepeds from Australia and from the 

 United States. In the Ascaridoidea, which occur in 

 land molluscs, fish, amphibia, reptiles, birds, and mam- 

 mals, one comes again to the problem of intermediate 

 host. All nemic parasites of fish and marine mammals 

 have intermediate hosts but intermediate hosts occur 

 in the development of ascaridoids parasitic in all other 

 vertebrate groups. The intermediate host is known 

 to occur only in one subfamily of the Ascaridoidea, the 

 Anisakinae. The family Cosmocercidae which is mor- 

 phologically the simplest (i. e., most rhabditoid) in- 

 cludes species that occur in amphibians and reptiles, with 

 occasional aberrant forms in snails. The Kathlaniidae 

 are more typically parasitic in reptiles, while the Heter- 

 akidae are reptile, bird and mammal parasites. Excluding 



reptiles while the Dracunculoidea and Spiruroidea are 

 parasites of fish, amphibians, reptiles, birds and mam- 

 mals, and the Filarioidea are parasites of all vertebrate 

 groups except fish. The latter (fish) are the lowest 

 normal final host of the Spirurida. Since all members 

 of the order require an intermediate host, it might be 

 logical to assume an aquatic origin for the group. How- 

 e.er, there is no reason for assuming the intermediate 

 host antedates the final host, but quite the reverse, 

 closely related species may have extremely diverse in- 

 termediate hosts according to the opportunities and 

 habits of the final host. If we were to suggest any group 

 as being similar to possible ancestors of the Spirurida, 

 it would be the Cylindrogasteridae (Rhabditoidea). 



Dioctophymatoids all have intermediate hosts; exam- 

 ples are known that parasitize only birds and mammals 

 in the adult stage while fish, amphibians, and reptiles 

 may act as secondary intermediate hosts. Evidently the 

 Dioctophymatoidea are primarily aquatic forms which 

 arose after the origin of birds and mammals — hence an 

 extremely recent group. All groups of vertebrates har- 

 bor trichuroids. Those that occur in aquatic animals 

 have intermediate hosts but species of Cajnllaria from 

 terrestrial mammals differ from one another in this 

 respec-t. One might suspect the Trichuroidea as being 

 as old as the Vertebrata but in that case one would 

 need to assume that the direct-Iife-cycle Trichurinae 

 (parasites of mammals) developed from the partially 

 indirect-life-cycle Capillariinae. As in the Dioctophyma- 

 toidea, one would trace them back to an aquatic nematode. 

 There is no clear evidence as to whether this nemic 

 group was marine or fresh water but from the fact that 

 ^peciation continues in land animals while marine spe- 

 cies are relatively less numerous, one could easily con- 

 ceive a fresh-water origin. Cystoopsis, a parasite of fish, 

 IS probably the most primitive genus. The Mermithoidea 

 are parasitic mainly in fresh water and terrestrial ar- 

 thropods (Tracheata) but do occur in Crustacea. Steiner 

 (1917) derived the Mermithoidea from the Dorylaimoidea 

 while Fiilleborn (1923) gave the same origin for the 

 Trichuroidea. Evidence from comparative anatomy in- 



194 



