NEMATODA 251 



fitting skin as a so-called "encysted" larva which possesses, however, 

 the power of movement. The protection of the cast skin and possibly 

 other factors enables this stage in the life history to resist desiccation 

 and to remain in a state of dormant metabolism until some odour of 

 decaying substances attracts the larvae and the opportunity of rapid 

 reproduction is given for a brief period. 



This third larval period is characteristically the period of wandering 

 in many nematodes, and this is seen in a remarkable manner in the 

 classical life history of Ancylostoma (Fig. 177). These animals hve 

 attached in the adult stage to the mucous membrane of the human 

 small intestine, sometimes in such numbers as to present an aspect 

 comparable to the pile of a carpet. They feed on the intestinal tissues 

 and only accidentally rupture the blood vessels, causing anaemia in 

 the host. The females are fertilized in situ and eggs are laid, which 

 begin to segment before they pass out into the faeces. The rest of the 

 life history may be shown as follows : 



(i) First larval form (rhabditoid) with a buccal cavity like Rhabditis. 

 This lives in the soil for three days before the first moult, which 

 produces the 



(2) Second larval form which moults after two days, the skin re- 

 maining as a cyst round this strongyloid larva (3). In this stage the 

 animal becomes negatively geotropic and thigmotropic, ascending 

 through the soil and being specially attracted to the moist skin of 

 human beings. This they penetrate by way of the hair follicles, though 

 occasionally the larva enters the gut by the mouth. In the former 

 event, the minute larva is able to make its way through the skin to 

 lymph spaces and to blood vessels, eventually being swept into the 

 circulation by the vena cavae to the right auricle, thence to the right 

 ventricle and then to the lung. In the pulmonary capillaries this 

 career is ended and the larvae make their way into the alveolar 

 cavities of the lung. They then travel by the bronchi and the trachea 

 to the oesophagus and so to the intestine. Here the animal is freed 

 from the second skin, producing the larva without buccal capsule. The 

 third moult produces the last larval stage towards the fifth to seventh 

 day and this is termed the larva with provisional buccal capsule (4). 



Finally, about the fifteenth day the fourth moult produces the worm 

 with the definitive buccal capsule (5), and in three to four weeks from 

 hatching the parasite has become sexually mature and is attached to 

 the epithelium of the intestine. - 



This most important human parasite shows in its earliest stages the 

 structure and the free-living habit of the primitive form Rhabditis, 

 and it is noteworthy that there are many species of the latter genus 

 which have already become parasites. 



It may, however, be supposed that a less specialized life history is 



