RHABDITIS-LIKE EMBRYOS. 61 



was sufficiently prolonged to allow of their taking in nutriment, 

 and so increasing in size. Eegarding the manifold conditions 

 under which the Nematoda lived, T thought it probable that 

 examples of this kind, if they existed at all, would be discovered 

 in this group. This opinion has been justified. My researches into 

 the life-histories of Nematodes, 1 have proved that there are numerous 

 species, especially among the Strongylidae (of human parasites 

 Dochmius duodenalis), which in their young stage resemble in 

 structure and habits the free-living Khabditidae (Figs. 42, 43), and 

 like them go on feeding and growing for a considerable time. They 

 then change their skin, lose the pharyngeal armature so very 

 characteristic of Rhabditis, and enter upon a stage when they cease to 

 take in nourishment and to increase in size, and need to become 

 parasitic. I need hardly recall the life-history of Ascaris nigrovenosa, 

 shortly described above (p. 2 ), which belongs to this type ; but is 

 peculiar, in that the Rhabditis-\\ke form, which elsewhere is merely a 

 young stage, is here developed into a special generation, which, as 

 soon as it is completed, enters again on a parasitic life. 



Among other Helminths (Cestodes, Acanthocephala, Distomidae) 

 there is nothing of the kind known, and it would indeed be impossible 

 in the two first-mentioned examples, inasmuch as the young has no 

 alimentary canal. Where there are free-living stages in these forms, 

 they serve only for an independent migration. Moreover, the Entozoa 

 are by no means the only animals which have a " swarm-period " like 

 this. It has often been observed in many other animals, such as 

 corals, ascidians, and so forth, when the adult is entirely stationary, 

 or possesses but limited powers of locomotion. Among the insects 

 also we know of wandering larvae, as Newport and Fabre have shown 

 in the Meloidae : the larvae of these beetles live in the nests of various 

 species of bees, to which they can only gain access in the young 

 condition, owing to limited powers of movement of the adults. 2 



As soon as the young parasite meets with its proper host, it 

 abandons its previous course of life, and loses those organs which 

 serve only to establish relations with the outer world, such as cilia, 

 locomotor appendages, and organs of sense, when these are present, 



1 For a fuller statement see Vol. II. 



2 The life-history of these young Meloidse is such an interesting example of pilfering, 

 that I cannot help giving an account of it here, especially as it affords many parallels and 

 points of relation to the study of parasitism. The females lay their eggs in early spring at 

 the roots of the Ranunculacese, dandelions, and other plants rich in honey, that are much 

 visited by bees. As soon as the larvae are hatched, they crawl up the stems of these 

 plants and hide themselves in the corolla. When bees visit the flowers the larvae attach 

 themselves to them by their powerful limbs, and are carried to the nest ; here they lose 

 their appendages and change into inactive grubs. 



