CHAPTER IV 

 LIFE HISTORY. GENERAL DISCUSSION 



B. G. CHITWOOD 



The dovclopmoiit of iioiiiiitodos in its sim|)lost form is diroct, 

 or not marked liy a motamorpliosis siu'li as occurs in tlio in 

 sects. In Ronoral tlio newly hatched nematode rosenil)les tlie 

 adult in all gross morpholoRic characters with the exception of 

 the reproductive system and secondary sexual characters. The 

 various growth stages, except the adult stage, are teruiinated 

 liy molts (or ecdysesi, the nunil>er of nuilts lieing four, the uum 

 her of stages five. Internal changes do not occur to any marked 

 extent in the simplest form of life history. We should, there- 

 fore, speak of the stages previous to the adult as nynijihs, if a 

 terminology were used similar to that emi)loyed in the Arthro 

 poda, but usage has made larva, as applied to such stages, the 

 accepted term. 



The number of uuilts occurring in the course of develojiment is 

 common for nearly all nematodes, and it ajipears to be the gen 

 eralized or primitive number for the class. I>evelopment may 

 be outlined as follows: 



First stage (larva) 



(molt) 



Second stage (larva) 



(molt) 



Third stage (larva) 



(molt) 



Fourth stage (larva) 



(molf) 



Fifth stage (adult) 

 Correlated with mode of life, various adaptations or modifi- 

 cations h,ive taken place in the life history, these adaptations 

 having arisen through the need for food and a means of dis 

 semination. With free-living nematodes, living either upon 

 decaying matter or preying upon other microscopic organisms, 

 these factors seem to have played a smaller part than with 

 those living as parasites. 



Probably need for dissemination was the earlier influence; 

 at any rate, it has caused the simplest modifications of life 

 history. The action of this factor on some free-living nema- 

 todes is evidenced by the occurrence of a persistent stage, the 

 cuticle of one of the larval molts being retained as a protec- 

 tive sheath or "cyst." It is not uncommon for such species to 

 have two types of larva, environmental conditions determining 

 whether or not larvae will be of the persistent type. The sig 

 nifieance of these persistent larvae is indicated by their nega- 

 tively geotropic tendencies, for tlie.v craw! to the highest sur- 

 face available and "standing" on their tails swing about, 

 catching upon any moving ob,iect. The climax of this type of 

 development is found in species where an encysted stage on 

 some arthropod (Rhabditis coarctata, see cover page, see. 1, part 

 1), or annelid is obligatory before the adult stage can be 

 reached. 



The need for obtaining food plays a much more striking 

 role, being evidenced by all conceivable degrees of parasitism 

 both on and in plants and animals. In the group of "herbi- 

 vors" life c.vcles may be of numerous t.vpes, depending on 

 whether tlie nematodes are "grazers," passing from host to 

 host, or sedentary forms, entering the host and there under- 

 going all or most of the stages of development. Life histories 

 may be further modified by the factor of dissemination and 

 the growth habits of the host. Among the jiarasitcs of animals 

 the factors of dissemination and nourishment also play their 

 roles. We have forms that are parasitic only during a par 

 ticular larval stage, the third, which, incidentally, is usually 

 sheathed or "encysted." Certain parasites of annelids (i.e., 

 Rhabditis pellio) pass the third stage in the nejihridia of their 

 host and can only develop to adults in the decomposing tissues 

 of their host. Other species (mermithids) enter their hosts 

 either as eggs or larvae and develop to ])readults (fourth stage) 

 within the body cavity, finally leaving their host before matur- 

 ing. In such instances the nourishment necessary for the entire 

 life cycle is obtained from the host and stored during the para- 

 sitic stages. 



The type of life history in parasitic nematodes being entirely 

 correlated with the degree of parasitism, we find, with more 

 ndvanced parasitism, more complicated life cycles and more 

 morphologic changes taking place during the course of devel- 

 opment. Seurat (Ullfi; 1920), recognizing this, proposed a ter- 

 minology for the different types of life cycles based on the 

 mode of development. 



Some forms have an alternation of generations, one genera- 

 tion being free living, the other parasitic. This type of life 



cycle is termed hctnopenonx. In such forms we find free-living 

 adults giving rise to larvae which enter the host and develop 

 to i)arasitic adults. These larvae may or may not be ensheatlied 

 (third stage), i.e., the cuticle of one or more larval molts re- 

 tained though separated from the body. The stage ready to 

 enter the host is ti'rnied the infrclivc stage. Nematodes with 

 no alteration of generations are termed monogcnous. These 

 are by far the most common, 



l'arasit<'s of animals may also be classified according to the 

 number of hosts necessary for completion of the life cycle. 

 Species in which there is a single host are termed monoxcnous, 

 those in which there are two or more hosts, hclcroxcnous. 



Some nematodes have both free-living and parasitic stages, 

 the free-living st;iges being larvae, the parasitic stages late 

 larvae and adults. In these we find young larval stages (the 

 first and second) feeding U|)on bacteria and similar organic 

 matter, the third stage usually ensheathed or persistent, this 

 commonly being the infective stage. Upon entering the host 

 these species develop through the fourth stage to the sexually 

 mature adult. 



A further development of parasitism is indicated by the ab- 

 sence of the free-living stages. Eggs of the parasite pass out 

 of the host and, outside, undergo only embryonic development 

 within the egg shell. In such instances the egg shell is often 

 covered by a protein layer (p. 178), and the embryo often con- 

 tains more yolk than forms in which the eggs hatch before en- 

 tering the host. With such a completely parasitic mode of 

 existence, the factor of dissemination again becomes manifest 

 and we find still other modifications in the life cycle. 



Some nematode parasites of vertebrates pass through larval 

 stages in invertebrates, this course of development being either 

 obligatory or facultative; still others undergo larval develop- 

 ment in other vertebrates, such development usually being obli- 

 gatory, rarely facultative. The host in which such a parasite 

 develops to infeetivity is termed the intermediate or secondary 

 host while that in which it develops to sexual maturity is called 

 the terminal, definitive or primary host. Sometimes the inter- 

 mediate host is eaten by another animal (secondary interme- 

 diate host) in which the parasite can continue its existence but 

 cannot reach maturity. When this second animal is, in turn, 

 eaten by the primary host the life cycle is completed. If the 

 parasite neither feeds nor undergoes growth within an animal, 

 that host is termed a transport Itost. This type of intermediate 

 host serves chiefly as a means of dissemination and is faculta- 

 tive rather than obligatory. 



We have attempted to extend Seurat 's outline to include all 

 nematodes. With the recent and extensive increase in informa- 

 tion on the life histories of vertebrate parasites it has become 

 very difficult to adjust Seurat 's classification to the many 

 variations in life cycles. For example it is hardly proper to 

 speak of a form as being heteroxenous when the use of an in- 

 termediate host is facultative {DictyocauUis filaria) yet other 

 nematodes in the same taxonomic group may be truly heteroxe- 

 nous requiring an intermediate host (Metastrone/i/his clonffa- 

 tus). In general one can say that the Spiruroidea, Filarioidea, 

 Camallanoidea, Draeunculoidca and Dioctophymatoidea are 

 heteroxenous. The Strongyloidea, Trichostrongyloidea and 

 Oxyuroidea are monoxenous while the Metastrongyloidea and 

 Ascaridoidea contain forms with both monoxcnous and heteroxe- 

 nous life cycles. Some e.xceptioiml forms do not fit into any 

 part of the classification. Ncoaplectana glaseri (Rhabditoidea) 

 and Probsimayria vivipara (Oxyuroidea) reproduce through 

 several consecutive generations within the host. Some strains 

 of a SIronpyloidcs species may reproduce without an alternation 

 of generations while other strains of the same species may be 

 predominantly heterogenctic. The diiBculty in fitting life his- 

 tories into a well defined classification appears to be due to the 

 adaptation of each species to its host which entails a means of 

 dissemination suitable to the host's environment and habits. 



A large assembly of nematodes have been found in more or 

 less close association with vertebrates or invertebrates. Some 

 of these merely use the "host" as a means of transportation 

 {Ehabditis coarctata which may pass an encysted stage on the 

 surface of dung beetles). Such nematodes are not considered 

 parasitic unless the.v actually penetrate the host. Some well 

 known free-living nematodes have been reported also existing 

 under parasitic conditions. Thus Rhabditis strongyjoides has 

 been repeatedly taken, in the larval stage, from diseased skin 

 of dogs and Diploscapter coronata from the ahydrochloric acid 

 stomaches of human beings. Yet these forms are free-living 



243 



