684 THE ANIMAL KINGDOM. 



Three stages accompanied by moults of the cuticle have been 

 distinguished in the growth of a Nematode after hatching the embryo, 

 larva, and the adult worm. The first is very small ; mouth, anus, and 

 digestive tract may or may not be discernible ; the oesophagus is pro- 

 portionately longer, and the sexual gland is present as a uni- or multi- 

 cellular rudiment. The tail is often singularly different to that of the adult, 

 and the head may be provided with a boring spine, lost in the next stage, 

 e. g. oral spine of Gucullanus. of Mermis> and ventral boring process of some 

 Ascarid larvae. The larva has the various organs distinct ; its tail 

 resembles that of the adult female, and has acquired sensory papillae some- 

 times of great size ; the genus and even species is discernible. Under the 

 larval skin the characters of the adult are assumed, and after a moult the 

 worm grows in size and matures genital products. 



The mode of life in the Nematoda is very variable, and no less than 

 thirteen distinct modifications of development are enumerated by von 

 Linstow as follows 1 : (i) The embryo developes directly into the sexual 

 animal, and inhabits fresh, salt, or brackish water, earth, plants, putrefying 

 substances, e. g. Enoplus and many other genera ; (2) the larva lives in earth, 

 the sexual form in plants, e. g. Tylenchtis Tritici in the ear of wheat, T. 

 putrefaciens in the Onion ; (3) the larva lives in worms, is set free by their 

 death and decomposition, and becomes sexual in the earth (Rhabditis pcllio] ; 

 (4) the sexual worms live in the earth ; the fertilised female enters different 

 species of Humble Bee (Bombtis] and the Wasp, passes into the coelome, pro- 

 duces ova which hatch out, and the offspring bore their way into the intestine, 

 and so escape (Spkaertilaria Bombi) ; (5) the larva lives in earth ; the 

 sexual form in a Vertebrate (Dochmius, Strongylus] ; (6) the worm lives as 

 a hermaphrodite in an animal, its progeny becomes sexual in earth, and the 

 progeny of the sexual animals enter the animal again (Anglos tomum^ e. g. 

 A. nigrovenosum, often called Ascaris or Leptodera nlgrovenosa, from the 

 lung of the Frog ; Rhabdonema strongyloides from the intestine of Man) ; (7) 

 a bisexual free form gives origin to a bisexual form parasitic in a Snail 

 (Leptodera appendiculata) ; (8) the egg is laid and passes into earth, gives 

 origin to an embryo which is transferred within the egg-shell to an animal, 



and hypo-blast : that its edges fold over, fuse from behind forwards, leaving a pore the future 

 mouth (Z. W. Z. xxvi. 1876). A similar mode of closure of the blasto- (gastro-) pore has been noted 

 in A. megalocephala and Angiostomum nigrovenosum. Gotte found an epibolic gastrula in the last- 

 named worm; a stomodaeum but no proctodaeum (Entwickelungsgeschichte der Thiere, i. 1882). He 

 also says that a proctodaeum has been observed by Ganin and Natanson in some Nematoda. Crley 

 found both a stomo- and a procto-daeum in Anguillula aceti, s. Tylenchus oxypliila (Monographic 

 der Anguilluliden, Buda-Pesth, 1880). In certain Oxyurids Galeb states that there is a delaminate 

 Gastrula, and that the whole digestive tract is derived from two ectodermic invaginations (A. Z. 

 Expt. vii. 1878, pp. 323, 368). Judging from structural anatomy alone, both oesophagus and 

 rectum would be said to be respectively a stomo- and procto-daeum. 



1 Z. W. Z. xlii. 1885, pp. 715, 716. Von Linstow reckons a i^th-mode, that characteristic of 

 Gordius ; see p. 687, post. The heads are kept as given by Von Linstow, but instances have been 

 added and a few slight changes introduced into the wording. 



