17'6 INVERTEBRATE MORPHOLOGY. 



single tube, the vagina, sometimes with muscular walls, whicli 

 opens to the exterior in the ventral mid-line some distance in 

 front of the anus. As a rule the sexes are separate, her- 

 maphroditism occurring only in a few isolated cases. 



Many Nematodes are free throughout their entire exist- 

 ence, living in the sea, fresh water, or damp earth, and fre- 

 quently possessing eyes. Others are found in some domestic 

 products, such as the vinegar-eel (Angiiillula), found in vine- 

 gar and sour paste ; while others, again, are parasitic on 

 plants, such as Tylenchus, which lives upon the young grains 

 of wheat and in some cases produces very serious damage to 

 crops, and Heterodera, which is quite as injurious to root- 

 crops. More interesting, however, are a number of forms 

 occurring as parasites in animals, many affecting man, in some 

 cases producing serious results. 



Life-histories of the Eunematoda. — The free-living forms show no 

 peculiarities of development, the immature animal developing directly 

 from the egg. Among the parasitic forms, however, interesting variations 

 from direct development, due to a change of host, occur, a well-marked 

 heterogony occasionally being found. An example of this is seen in Bh-ab- 

 ditis nigrovenosa, which at one stage of its existence lives in damp earth, 

 the females being viviparous and producing young which make their way 

 into the lungs of frogs, where they assume a form which led them to be 

 assigned to the genus Ascaris, and where they become mature. At this 

 stage they differ from the Rhdbditis forms in being hermaphrodites, and 

 from the eggs deposited by them the Rhabditis generation again results. 



From a medical standpoint one of the most important forms is Trichina 

 spiralis, which occurs encapsuled in the muscles of various warm-blooded 

 animals, such as man, the pig, rat, mouse, and occasionally in the fox, cat, 

 and rabbit. The capsules are oval and about 0.6 mm. in length, and occa- 

 sionally have a white color, due to the deposition of calcareous matter in 

 the wall. In the interior of the capsule lies coiled up an immature 

 Trichina, which may retain its vitality in this condition apparently during 

 the lifetime of its host. Should, for instance, improperly cooked or salted 

 pork which contains such capsules be eaten by man, the capsule becomes 

 dissolved by the digestive juices and the young Trichina is set free in the 

 small intestine and in the course of a few days becomes sexually mature. 

 Each female may deposit in the intestine as many as 1000 eggs, from 

 which, in the second or third week after infection, young Trichince meas- 

 uring about 0.01 mm. hatch out and at once proceed to bore through the 

 walls of the intestine, producing a more or less violent inflammation 

 according to the degree of infection. They wander through the connective 



