208 



L WER IN VER TEBRA TES. 



mature worms. But after a time they are seized with a migratory instinct, and crawl off 

 in all directions, the young worms with the rest. In the drier earth the young slough 

 their skin, but do not leave it ; on the contrary, it forms a protective case, which the 

 larva inhabits until it again encounters moisture and i)utrefaction, when it bursts from 

 its imprisonment and renews the cycle of life. According to Schneider also, some 

 other species of these genera occasionally exchange theh- free life for parasitism upon 

 the large black field snail of Europe and upon the common earth-worm, but in favora- 

 ble external surroundings leave their host again. It is but a step to change from this 

 habit to the true parasitic life led by most Nematoda. 



The Rhabditis ■)ugi-ocenosa is a parasitic membgr of the family of Anguillulidse, 

 and has a most remai-kable life history. There are two generations of sexually mature 

 animals. The members of one generation are very tiny, perhaps two hundredths of 

 an inch long, lead a free life in moist earth or mud, and resemble anatomically the 

 Leptodera. The members of the second generation grow to half an inch long, inhabit 

 the lungs of frogs, and were formerly supj)osed to belong to the Ascaridse, a very dis- 

 tinct family. Evidently something is wrong, — either our system of classification or 

 else nature itself. Let us trust to the future for the resolution of our dilemma. The 

 first generation is developed from the eggs of the first, and vice versa. The eggs from 



Fig. lyo. — A/ii/iUllula trUicl, wheat woriii 



the parasitic form are discharged through the intestines of the host, and develo]) 

 directly into the free form ; the female of the latter, however, retains the eggs, and 

 the young develop within the parent. " She exceeds the pelican, for the mother nour- 

 ishes her offspring not only with her blood, but all her internal organs break up until 

 nothing remains but the skin, to form a lifeless case around the brood of squirming 

 young worms." This stage of life lasts some time, and then the young jump out and 

 remain perhaps for weeks in the moist earth, until they have an ()])portunity, by way 

 of a frog's mouth, of getting into his lung and there growing u]) into the Ascari 

 ni(jrovenos(je. 



Other members of the Anguillulidse are very injurious to plants. The most noto- 

 rious of these enemies to husbandry is the Anrjidllula tritici (known also under the 

 name of Tylenchus tritici), for it inhabits find destroys the ears of wheat, producing 

 the dreaded disease known as smut. These worms were discovered about the middle 

 of the eighteenth century by the English microscopist Needham, but our present 

 knowledge of them is due mainly to Monsieur Davaine. In the diseased ears the grains 

 are malformed, small, and black ; they have a hard crust, which encloses a coi-e of 

 white powdery substance, which upon being moistened falls into little bodies, 

 which the microscope shows to be Anguillulffi, but still immature. In nature the 

 kernel of wheat falls to the ground, gets moist and rotten, the wall breaks open, 



