Ehabdiasidae 



The niembers uf tlio genera EJiabdian and Entoinclat:, now 

 separated into a separate family from Strunyyloides, resemble 

 Strongyloididae in having an alternation of generations, at 

 least in some species. This double life cycle was tirst demon- 

 strated by Meczuikov (18G.j) in the case of M. btifonis. Unlike 

 StrongyluidcK, the parasitic generation, at least of some species 

 of Sliabdias, consists of hermaphroditic females, possessing a 

 well developed seminal receptacle. Seurat (1920a), however, 

 thinks that the parasitic forms of Entumdas dujardini and 

 E. entomclux from Anguis fragilis are parthenogenetic rather 

 than protandrous hermaphrodites, since he was unable to find 

 seminal receptacles or to detect sperms. 



As in Strongyloides, l)oth homogonic and lieterogonic types of 

 development may occur in the free-living phase of the life cycle 

 of Sliabdia.i. In most of the species one type or the otiier 

 strongly predominates or may even occur exclusively, though 

 in some of the forms in which one type of development was 

 long thought to occur exclusively, the alternative type has 

 more recently been observed. Travassos (i;i2()) called atten- 

 tion to the fact that the species found in Amphibia and La- 

 certilia have indirect development, while those found iji snakes 

 liave direct development. Chu (iy3(i), however, reported some 

 unpublished observations of Chitwood 's, and also some of his 

 own, in which both types of development were found in sev- 

 eral amphibian and reptilian species, (ranac, eustreiitus, fulle- 

 borni, and fuscovenosa var. caUtiicnfiin). In the last-named spe- 

 cies Chu observed only homogonic development except wlien 

 an especially favorable culture medium was used, whereupon 

 a small percentage of free-living adults, predominantly males, 

 were usually found. The oflspring of these adults failed, how- 

 ever, to infect snakes. It seems evident from this data that the 

 course of development of Bluibdia!< is determined by factors 

 similar to those operating in the case of Utroiigijloidcs. 



Whereas in SI niiigiituidc s both direct and indirect infective 

 larvae are filariform, in the Rhabdiasidae the direct huvae are 

 rhabditiform while the indirect ones are filariform (cf. Figs. 

 17ii, P-Q). The free-living adults of different species vary con- 

 siderably in their mode of reproduction. Travassos describes the 

 free-living female of S. fiiUcborni of frogs as producing only 

 one or two larvae, which may become fully developed within the 

 mother, destroying her tissues, whereas Chu (lit.^G) describes 

 K. fuscovenona var. cataiiciiin as having a few eggs in each horn 

 of the uterus, which are usually laid when little or no develop- 

 ment has occurred. 



According to Goodey (,l!t24) the homogonic larvae of S. 

 fuscovenosa undergo two ecdyses outside the body of the host, 

 the second shed cuticle being retained as a tight-fitting sheath 

 for the infective larvae. The sheath is shed' upon gaining 

 entry to the host. The larvae molt twice more during de- 

 velopment in the host's parenteral tissues, but both shed 

 cuticles are retained as sheaths. Although the infective larvae 

 of S. bufoiiis were reported by FuUeborn (1920) to penetrate 

 the skin, and by the same writer (1928) to migrate to the 

 lungs via the circulatory system, Goodey (1924) failed to 

 get the infective larvae of R. fuscovenosa to penetrate skin, 

 although their behavior outside the body was like that of 

 skin-penetrating larvae, and he also thought it probable, from 

 their distribution in the body, that they migrated to the lungs, 

 after penetrating the gut wall, by direct migration through 

 the mesentery and not via the blood stream. Fiilleborn (192S) 

 called attention to the fact that larvae of E. bufoiiis would 

 also penetrate snails and possibly other invertebrates, where 

 they rcmaiu unchanged for weeks, capable of infecting a frog 

 when the snail is eaten. Similarly the larvae may sometimes 

 became encapsulated parenterally in frogs which may then 

 act as "transport hosts" for infection of larger frogs which 

 eat them. Fulleborn suggests that since the skin of snakes is 

 hard to penetrate transport hosts may constitute the principal 

 method of infection for these hosts. 



STRONGYLINA 

 I. STRONGYLOIDEA AND TRICHOSTRONGYLOIDEA 



Three geneial types of life cycles, which more or less merge 

 into each other, occur in the superfamilies Strongyloidca and 

 Triehostrongyloidea of the suborder Strongyliua. One of these, 

 characteristic of the Ancylostomatidae and a few other forms 

 in the Strongyloidca and Triehostrongyloidea, is essentially 

 the same as the homogonic cycle of Ehahdias, except that the 

 parasitic worms are bisexual. It involves development to the 

 third (infective) stage outside the body of the host, skin pene 

 tration, and parenteral migration via the circulatory system 

 after infection. The second, characteristic of most of the 



Triehostrongyloidea and many of the Strongyloidca, differs 

 in that there is no skin penetration, and no migration in the 

 host beyond the walls of the alimentary canal. The third, 

 characteristic of the Syngamidae in the Strongyloidca, in- 

 volves development and molting within the egg, w^ithout feed- 

 ing or growth, with at least optional establishment in an in- 

 vertebrate transport host, and a parenteral migration which 

 leads to the respiratory system but not lieyond. We think it 

 probable that both types 2 and 3 were derived from type 1, 

 although it is also possible that type 2 is the most primi- 

 tive, and that types 1 and ?> were both derived from this. 



1. Ancylostoma spp. 

 The genus Ancylostoma will serve as a typical example of the 

 first type, involving skin penetration and parenteral migration. 

 The eggs of these worms are deposited by the adult females 

 in the lumen of the small intestine, whence they make their 

 exit with the feces; at the time of leaving the body of the 

 host they are nearly always in the four-celled stage of devel- 

 opment, normally being unable to progress beyond this point 

 without free oxygen. Under optimum conditions of oxygen, 

 moisture and warmth (7.1° to 8.')° F.) the eggs proceed with 



Fig. 181. DEVELOPMENT OF HOOKWORMS 



Ani-ylostonui dtindenale. A — First stage larva ; B — Second stage; C — 

 'I'liird stage. D-H — Development of primitive and definitive capsules; 

 ( 1) — Bladder-like structures forming around the larval oral cavity, with 

 heginning of formation of the primitive larval teeth; E — Nearly com- 

 pleted primitive capsule, with triangular teeth at base, and old larval 

 oral cavity still running through center of primitive capsule; F — Fully 

 developed primitive capsule with beginning of formation of bladders at 

 its base; G — Later stage in development of dorsal and ventral bladders 

 which will eventually form the definitive capsule; H — Later stage in 

 development of definitive capsule, with primitive capsule still connected 

 with esophagus by a strand of tissue; I — Female larva with definitive 

 capsule formed but primitive capsule still attached ; J — -Male after final 

 moult but last cuticle still enclosing it; K — Male larva with primitive- 

 capsule. After Looss. Chandler. A. C. 1929, Hookworm Disease. 



272 



