426 PHYSIOLOGY OF THE INVERTEBRATA. 



system,* The nephriclia or segmental organs begin as out- 

 growths from the posterior face of each septum ; and finally 

 the adult form is attained just before the hatching of the ^^^' 



The Polychwta are dioecious and rarely hermaphrodite. t 

 Some of these animals multiply by fission and gemmation. 

 In ^yllis^ transverse fission takes place ; while in Myrianida 

 gemmation occurs giving rise to zooids. In some other 

 Polych^tous forms reproduction is produced by a combination 

 of fission and gemmation. 



The genital organs of the Polyclwta are very simple in 

 structure, and the genital products ultimately float about in 

 the perivisceral cavity, probably passing outwards through the 

 apertures at the bases of the parapodia. In some Polycliceta, 

 the nephridia act as genital ducts. The ova of these animals 

 undergo a somewhat similar metamorphosis to those of the 

 Oligoclicda and Hirudinca ; " but the embryos of the Polychcda 

 differ from those of the Oligochceta and Hirudinca in being 

 ciliated." 



The Nematoscolices. 



The Ncmatoidca are nearly all dioecious, unsegmented 

 worms. + These animals and their genital products are pos- 

 sessed of great vitality. According to M. Devalue, the ova 

 of Ascaris lumhricoidcs § are capable of withstanding the 

 action of a solution of chromic acid (2 per cent.) for five 

 years; and Mr. W. CarruLhers, F.R.S., states that vitality 

 was restored in some Nematodes, after they had been in the 

 botanical department of the British Museum for more than 

 thirty years. || 



* The nervous sj-stem, almost throughout the animal kingdom, has an 

 epiblastic origin. 



t Protula is hermaphrodite. 



+ 7'eW//^e.s' is hermaphrodite ; and A^caria nirjrovenosa, which at first pro- 

 duces spermatozoa, afterwards only produces ova. 



§ The ^Xnii'Q (Trfioyyi'Xof of Hippocrates. 



II See Dr. Griffiths' Diseases of C'roj)s, pp. 18 and 119. 



