istic one. While most forms possess a digestive tube there are 

 marked indications of its reduction, and in the Acanthocephala the 

 tube is lacking. Some roundworms are free-living, others parasitic, 

 the latter including forms parasitic only in larval or adult stage of 

 the life history, and also facultative parasites. Most parasitic 

 forms are intestinal, but important species occur in the muscles 

 and connective tissues of the host. There is frequently some 

 device for retaining position, such as thread-like or lance-like form 

 or development of hooks. 



Nematoda, typical round or thread worms, include a striking 

 variety of forms and life histories, some of the more significant of 

 which are described below. One of the oddities of the group is 

 Anguillula aceti, the vinegar "eel", the female of which is 2.5 mm. 

 in length, the male smaller. The organism lives in ordinary 

 vinegar, subsisting on the fungus sometimes present. Similar 

 forms live in soil or decaying matter. 



The type of the group is commonly taken from Ascaris, the 

 species of which are parasitic in the intestinal tube of various verte- 

 brates, A. megalocephala occurring in the horse, A. suum in the pig, 

 and A. lumbricoides in man. Ascaris lumbricoides is a yellowish 

 cylindrical worm, the male 15 to 17 cm., the female 20 to 25 cm. 

 in length. There are four longitudinal lines, respectively dorsal, 

 ventral and lateral, the former two containing nerves and the latter 

 excretory vessels. The mouth at the anterior end is enclosed by a 

 trilobed lip. The genital aperture in the female is ventral, about 

 one-third of the distance back from the head; in the male posterior 

 and terminal, the end of the more slender body being curved 

 ventrally and the aperture marked by the penial setae. There is a 

 complete digestive tract. The muscles of the body wall are of a 

 peculiar and in some respects primitive type, being transitional 

 between protoplasmic cells and fibres or showing the features of 

 both. The ovaries are paired and communicate with the outside, 

 in the position indicated, through an unpaired vagina. The male 

 gonad is a single thread opening posteriorly through a seminal 

 vesicle into the posterior end of the intestine. Development is 

 direct but there is a migration of the young from the intestinal 

 cavity, by way of the portal system, to the heart and lungs, fol- 

 lowed by a return through the respiratory tubes and oesophagus 

 to the intestine. Infection takes place through the water from the 

 soil. Occurs most commonly in young children. 



Oxyuris vermicularis, a minute pin-worm parasitic in the 

 human intestine, is typical of a group of worms having a pair of 

 lateral flange-like extensions of the cuticle, which in the female 



11 



