NEMATELM1NTHES. 311 



in neither case however do they undergo important changes unless eventually 

 swallowed by a frog. They then pass down the ti-achea into the lungs and 

 there rapidly develop into the adult form. No separate males have been 

 found in the lungs of the frog, but it has been shewn by Schneider (No. 390) 

 that the so-called females are really hermaphrodites; the same gland giving 

 origin to both spermatozoa and ova, the former being developed before the 

 latter 1 . The remarkable feature of the above life history is the fact that in 

 the stage corresponding with the free larval stage of the previous forms 

 the larvse of this species become sexual, and give rise to a second free larval 

 generation, which develops into the adult form on again becoming parasitic 

 in the original host. It constitutes a somewhat exceptional case of hetero- 

 gamy as defined in the introduction. 



Amongst the Nematodes with but a single host a remarkable parasite 

 in wheat has its place. This form, known as Anguillula scandens, inhabits 

 in the adult condition the ears of wheat, in which it lays its eggs. After 

 hatching, the larvse become encysted, but become free on the death of the 

 plant. They now inhabit moist earth, but eventually make their way into 

 the ears of the young wheat and become sexually mature. 



The second group of parasitic Nematodes with two hosts may be 

 divided into two groups, according to whether the larva has a free 

 existence before passing into its first or intermediate host, or is taken 

 into it while still in the egg. In the majority of cases the larval forms 

 live in special connective tissue capsules, or sometimes free in the 

 tissues of their intermediate hosts ; but the adults, as in the cases of 

 other parasitic Nematodes, inhabit the alimentary tract. 



The life history of Spiroptera obtusa may be cited as an example of a 

 Nematode with two hosts in. which the embryo is transported into its 

 intermediate host while still within the egg. The adult of this form is 

 parasitic in the mouse, and the ova pass out of the alimentary tract with the 

 excreta, and may commonly be found in barns, etc. If one of the ova is 

 now eaten by the meal-worm (larva of Tenebrio), it passes into the body 

 cavity of this worm and undergoes further development. After about five 

 weeks it becomes encapsuled between the 'fat bodies' of the meal-worm. 

 It then undergoes an ecdysis, and, if the meal-worm with its parasites is now 

 eaten by the mouse, the parasites leave their capsule and develop into the 

 sexual form. 



As examples of life histories in which a free state intervenes before 

 the intermediate host, Cucullanus elegans and Dracunculus may be se- 

 lected. The adult Cucullanus elegans is parasitic in the alimentary tract 

 of the Perch and other freshwater fishes. It is a viviparous form, and the 

 young after birth pass out into the water. They next become parasitic in 

 Cyclops, passing in through the mouth, so into the alimentary tract, and 

 thence into the body cavity. They soon undergo an ecdysis, in the course 

 of which the oesophagus becomes divided into a muscular pharynx and true 

 glandular oesophagus. They then grow rapidly in length, and at a second 

 ecdysis acquire a peculiar beaker-like mouth cavity approaching that of 

 the adult. They do not become encapsuled. No further development of 



1 Leuckart does not appear to be satisfied as to the hermaphroditism of these forms ; 

 and holds that it is quite possible that the ova may develop parthenogenetically. 



