FREE-LIVING NEMATODES EHABDITID^E. 



95 



globularly expanded portion (the so-called "Bulbus") an armature 

 usually formed of three valvular teeth (Fig. 60). Sexual maturity is 

 attained only through abundant nutrition, mostly only in places where 

 a mass of decaying matter has been formed. 

 In such localities the generations follow upon 

 one another often so closely, that the young- 

 worms may be found there in large numbers and 

 in all stages of development. When this decay- 

 ing matter ceases to exist, either through being 

 exhausted or dried up, then the creatures scatter 

 and continue in the larval state, until some 

 favouring fortune grants them the possibility of 

 further development. In this young state, pro- 

 vided with a cystic larval membrane (with oc- 

 cluded mouth and anus), they can withstand 

 desiccation for a considerable time without perish- 

 ing. Under certain circumstances these mouth- 

 less larvae reach the interior of living animals, 

 where they then, evidently in consequence of 

 their parasitism, enter upon a course of develop- 

 ment which differs considerably from their usual 

 life-history. This is specially the case with a 

 species which was first described by its dis- 

 coverer, Schneider, under the name Alloionema 

 appendiculatum^ though he has more recently FIG. 

 correctly recognised it as a Ehdbditis (Lepto- 

 dera). 2 The researches of Schneider, and more especially of Glaus, 8 

 show that the parasitism of this interesting form is a purely optional 

 one, and that it can be abandoned without change of its specific 

 characters. In the latter case the life-history follows the ordinary 

 course ; but it is otherwise when the larvae have the opportunity 

 of migrating into the black slug (Arion ater). In this they de- 

 velop into animals which reach double their size (over 4 mm.), not- 

 withstanding the absence of a mouth ; they also lose the chitinous 

 oesophageal teeth and awl-shaped caudal point they formerly pos- 

 sessed, but there develop instead two finely streaked long cuticular 

 bands at the posterior extremity of the body, whose function is most 

 probably that of organs of touch, seeing that they occur also in other 

 Nematode larvae in this position. 4 The parasites, however, attain 



1 Zeitschr. f. wt'ss. Zool, Bd. x., p. 176, 1860. 



2 " Monographic der Nematoden," p. 159 : Berlin, 1866. 



3 " Beobachtungen liber die Organization and Fortpflanzung von Leptodera appen- 

 diculata :" Marburg u. Leipzig, 1868. 



4 See Vol. IT. 



