3/6 METAMORPHOSIS. 



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 larva; 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 heterogamy 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 larvae 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 cap- 

 sules, or sometimes free in the tissues of their intermediate 

 hosts ; but the adults, as in the cases of other parasitic Nema- 

 todes, 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 selected. 

 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 



1 Leuckart does not appear to be satisfied ns to the hermaphroditism of these 

 forms ; and holds that it is quite possible that the ova may develop parthenogeneti- 

 cally. 



