(1) Swoiul niiil siilisf(nicnl iiiti'iiiu'ilinli' liosis <i|ilii)ii:il, 

 I'ossililo Yai'iatioii in nmiiy siilnlivisions aliovo. 



(2) Two snccfssivo inlormodiali' hosts (ililiRatorv : deli- 

 iiitivi' host roHchoil t)V ciitiiiK <•' second inti'inu"- 

 diiito host. Kx., (liiutlio.itoma. 



RHABDITOIDEA 



As noted on p. 2li7, thi'io iiro a consideialilo nnnilur iif mnui 

 todes IirlonKini; fo this pi-oiip whieh are faenltative parasites 

 of verteliiates, but only the Klialidiasidae and StioiiKyloididae 

 have heeonie obliflator;/ parasites of vertelrrates. In Imtli oC 

 these families there is a tendeney for an alternation between 

 f ree liviiiB and parasitic Renerations, and in both, e.xeept in 

 Olio species, I'ara.i't ronni/loitlts itiiichesi, there is a sn|)pre.ssion 

 of males in the parasitic H''">''''''io"- The larvae of most sjie 

 cies pellet r;ite the skin or mucous membranes and iiiinrate to 

 the luntis before trrowiiin to maturity. The Rliabdiasidae nia 

 ture in the luiiKs. and this is probably the mure ]iiimitive 

 condition; the StrouKyb'ididae only exceptionally mature in 

 the luiiKs, ordinarily returiiiiiK to the intestine before ma- 

 turinj;. 



Stronuvi,oidii).\k 



The life cycle of Stronmiloidis stercoral iv of man. the main 

 features of which were first elucidated by Grassi (1S78) and 

 I>euckart (1S.S:;), has been studied by a large number of 

 proniiiient parasitologists, yet even today there is no unanimi 

 ty of opinion about some jihases of it. Grassi observed di 

 roct development into filariform* larvae; Leuckait discov 

 ered that an alternation of gener.-itioiis might occur; van 

 Dunne (ISH):;) first demonstrated that infection resulted from 

 skin iH-netration ; and Looss (liH)")) showed that the migrntimi 

 of the larvae after penetration paralleled that of Ancylo.iloiixi. 

 Important additional details or interpretations w^itli resjiect 

 to this or related species have been added by Leiclitensteni 

 (lS9i>. 190.-)), Fiilleborn (1!>14), Sandground (lltiii). Xislii- 

 gori (1!»2S), Kreis (1>»32), Faust dilHS"), Liicker (l!t.S4), 

 Graham (1!).'?6-I03i1), Beach. T. 1)., n!l3.5 193(i) and Chitwood 

 and Graham (liHO). 



The parasitic females live more or less deeply imbedded in 

 the mucous membrane of the small intestine where they pro 

 duce embryonated eggs which in this species hatch promptly 

 within the host. The embryos are rhabditiform, and resemble 

 those of hookworms except for the very sliort stoma. They nor 

 mally pass out of the body with the feces of the host, and 

 then begin to feed and grow. From this point on they may 

 follow either o le of two courses of development, known re 

 spectively as the direct or liomogonie type, and the indirect 

 or heterogoiiic type. 



In the homogonic type of development the rhabditiform 

 larvae grow and transform into filariform larvae, sometimes 

 in 24 hours or less. Looss reported two molts in the course of 

 the develoiunent of rhabditiform to filariform larvae in the 

 human and other species, and Lucker (1934) observed two 

 molts in the larvae of S. ranxoiiii of pigs, but other observers 

 have not mentioned more than one molt. The -second stage 

 larva, according to Lucker. does not at first differ in any 

 morphological characters from the first stage larva, but transi- 

 tion to the filariform type of larva begins soon afterwards. 

 The first molt occurs 12 to 18 hours after hatching, the sec- 

 ond within 48 hours, at room temperatures. The filariform 

 larvae are unsheathed and constitute the infective stage. They 

 creep up on points of vantage in the soil or culture, often 

 clustering together in brush like groups to await an opportu- 

 nity to burrow through the skin of a host. 



In the heterogoiiic type of ilevelo])meiit the rhabditiform 

 larvae, instead of developing into filariform larvae, change 

 into adult free living males and females. With the exceptimi 

 of Lucker (1934), no observer has mentioned or suggested more 

 than a single molt in the course of this development, but 

 Lucker has been able to trace the usual four molts and five 

 stages. The only morphological changes are in size and in 

 growth of the genital primordiiim until the fourth stage is 

 reached, when the structure of the head simulates that of the 

 adult, and the male and female characters are gradually as 

 sunied. At the time of the final molt the ovaries of the females 

 and spicules of the males are fully formed. Adults may be 

 found after 36 to 48 hours at room temperature. 



The impregnated females produce eggs soon after they 

 reach maturity, and these hatch soon after being deposited. 

 These first-stage rhabditiform larvae are morphologically simi 

 lar to those hatching from eggs deposited by the iiarasitic fe 

 males, although a few small differences have been mentioned 



*The term "filariform** is used here to denote the third stage larva of 

 Stronffyloitles to distinguiKh it from the third stage larva of strongyliiis 

 which is called "slrongyliform** larva. Its use in no way signifies a 

 similarity to any slage of tilariids. Actually, the esophagus is very 

 similar to that of an infective strongyl larva. — B. O. C. 



by Kreis (1932). AIIIi.mikIi Kieis. like all ollicis lirfure him, 

 fails to niention luore than a single molt, l.ucker was able to 

 trace the orthodox two molts before the infective filariform 

 larva was iirodnced, .just as in the case of filariform larvae of 

 direct devi'lopmeiit. No dilTercnce between the two types of 

 infective larvae has been noted. 



Heacli (193r) 193(i) showed that under particularly favorable 

 conditions there may be several generations of free living 

 bisexual forms. Kouri, Hasnuevo and .Xrenas (1930) reported 

 that .S*. sicrciirnli.s, after numerous free living generations, be- 

 comes entirely free living; the females become iiarthenogenetic 

 and there are no males, but the fecundity of the females gradu- 

 ally decreases until the cultures become sterile. 



.Nishigori ( 1928) first demonstrated the opposite extreme in the 

 life cycle of .S*. sti-rroriiliK — internal auto infection (called hyper- 

 infection by Faust), with complete eliinination of a free liv- 

 ing stage. Nishigori also suggested circumstances under which 

 this might occur. Faust (1931) and Faust and Kagy (1933) 

 continued Nishigori 's observations, but the evidence was in- 

 conclusive for many until Faust and detiroat (1940) made ob- 

 servations at autoiisy of a case which left no room for doubt 

 but that under exceptional conditions in human beings auto- 

 infection by filariform larvae of S. nlircoralis through the walls 

 of the colon can occur. There is no certainty, however, that 

 it occurs in other siiecies or hosts. 



The filariform larvae of S. .stfrc(ir<ili« and most other sjiecies 

 normally jienetrate the skin. If swallowed they burrow through 

 the mucous niembranes of mouth, esojihagns or stomach. Miiii- 

 nig (1930), however, states that sheep are usually infected 

 with S. papilloxiiK by mouth, this species being a poor skin-penc- 

 trator, and that larvae administered by month do not migrate 

 to the lungs. Lucker 's (1934) experiments with .S. ransiimi 

 suggest that in this species also migration to the lungs may 

 not take place after oral infection. 



The larvae of .S*. xhrcorallx, after penetrating skin or mu- 

 cous membranes, enter the circulatory system and are carried 

 to the lungs. Faust (1933) states that they reach the lungs 

 unchanged; they are sometimes recovered as earl.v as the third 

 day, and sometimes as late as the thirtieth day. Although no 

 molts are mentioned, Faust distinguishes iiost-filariform, pre- 

 adolescent, adolescent, aid mature female mid male forms. The 

 post-filariform type of larvae is found most commonly in the 

 lungs about the fifth day; if carried to the digestive tract 

 they seem unable to establish themselves. These larvae are 

 slenderer than infective larvae, with longer esophagus, and 

 are more plastic. The preadolescent forms also occur princi- 

 pally ill the lung tissue and bronchioles, and are believed to be 

 too immature to establish themselves in the intestine. At this 

 stage, according to Faust, sexual differences are observable 

 for the first time, the female being still more slender than the 

 post filariform type and with a longer esophagus, whereas the 

 male shows decided resemblances to the rhabditiform larva. 

 The adolescent forms are migratory, and are commonly found 

 not only ill the lungs but also in the upper parts of the respira- 

 tory ti-ee, e.sophagus and intestines. Both mature females 

 and males were reported from lung tissue and bronchioles, but 

 only mature females from the intestine, where they burrow 

 into the walls. Lucker (1934), studying S. ransomi, ob.servcd 

 only a single molt after entering the body of an animal, this 

 occurring in the intestine about 6 days after infection ; Looss 

 (1911) also reported only a single molt, but Fiilleborn (1914) 

 apparently considers that two molts occur. By analogy with 

 other nematodes, and with the development of the free living 

 adults of atrongyloidcs itself, it would seem more probable that 

 two molts do occur in the course of the development in the 

 host. 



There has been much difference of opinion on several points 

 in connection with the life cycle of Strimt/yloides, particularly 

 (1) the factors determining whether the development is homo- 

 gonic or heterogoiiic; (2) the reproduction status of the jjara- 

 sitie females, and (3), since the work of Kreis (1932) and 

 Faust (1933), the occurrence and function of parasitic males. 



Sandground (1926) gave a brief but valuable summary of 

 views up to the time of his writing on the factors determining 

 direct or indirect development. Environmental factors were 

 first thought to be the cause, but Braun (1S99) and others 

 showed that such was not the case; Sandground felt that there 

 remained no substantial rea.soii for questioning the gener- 

 ally accepted idea that the direction of development was fixed 

 before the larvae entered their period of free life. Leichten- 

 stern (190.-|) advanced the view that there were two genetically 

 different varieties of the human species, differing in their life 

 cycles, the indirectly developing variety being confined to the 

 tropics, the directly developing one being especially character- 

 istic of the temperate zone. Leichtenstern considered the 

 heterogonic type to be the more primitive and gave a very 

 jilausilile explanation for the evolution of the homogonic type. 

 Darling (1911) suggested as a cause environmental effects on 

 the rhabditifonn larvae jirior to leaving the host, and Brumpt 



269 



