j,^ ROUNDWORMS. PHYLUM NEMATODA 



waiuloring stage, is reached in about a fortnight. It remains 

 witliin the second larval skin, and can penetrate through the skin 

 of an>' part of the body of man. The foot is a common site in 

 countries where shoes are not worn, and larvae on salads or in 

 drinking water may enter through the membrane of the mouth. 

 Having pierced the skin, the larva enters a venule or a lymphatic, 

 and goes by the veins to the right side of the heart and to the 

 lung. It is too large to enter the capillaries, and so pierces the 

 lung walls and passes into a bronchiole, whence it crawls up 

 the trachea and is swallowed. It acquires a temporary capsule 

 in the stomach, and after two more moults is adult. The hooks 

 by which it attaches itself to the intestinal wall are acquired 

 at the third moult. The period from infection to the appearance 

 of eggs in the fseces is about eight weeks. 



4. Larva: parasitic, adults free. — This is a reversal of the common 

 state of affairs. Mermis nigrescens, the Rain Worm, also illustrates 

 the Nematoda parasitic solely in invertebrates, although there 

 are others in which the life-cycle is more normal. The adults 

 live deep in soil and pair there ; the larvae penetrate the skin of 

 insect larvae, and develop in the body cavity until they are 

 sexually mature, when they return to the soil. The worm gets 

 its English name from the peculiar habit of the adults of climbing 

 the stems of plants after summer rain in such numbers as to give 

 rise to the legend of ' showers of worms '. 



5. LarvcB and adults parasitic in different animals, with a free 

 stage. — An example is the Guinea Worm, Dracunculus medinensis, 

 the female of which may be three or four feet long. The posterior 

 part of the gut is lost and there is no anus, and almost the whole 

 body is occupied by an expanded uterus containing larvae. These 

 escape, either through a female genital opening or vulva just 

 behind the mouth, or through the mouth itself, while the host is 

 immersed in water. The larvae must penetrate a crustacean of the 

 genus Cyclops (page 221), and here they develop for three to five 

 weeks, with two moults. If a man drinks water containing infected 

 Cyclops the larvae are released, and bore their way to the sub- 

 cutaneous tissue. As the worm grows it goes towards the extrem- 

 ities, especially the ankle, and here the head comes near the surface 

 and is exposed in a small ulcer. It is the contact of cold water 

 with this which stimulates the liberation of the eggs. The male is 

 shorter than the female and has rarely been seen. The worm 



ts man in many tropical countries. 



