534 



ANNULOSA, OR WORMS. 



FIG. 273. 



to it. The cavity of the stomach does not give origin to any in- 

 testinal tube, nor is it provided with any second orifice; but a 

 large number of ramifying canals are prolonged from it, which 

 carry its contents into every part of the body. This seems to 

 render unnecessary any system of vessels for the circulation of 

 nutritive fluids; and the two principal trunks, with connecting 

 and ramifying branches, which may be observed in them, are 

 probably to be regarded in the light of a "water-vascular" 

 system, the function of which is essentially respiratory. Both 

 sets of sexual organs are combined in the same individuals; 

 though the congress of two, each impregnating the ova of the 

 other, seems to be generally necessary. The ovaria, as in the 



Entozoa, extend through a large 

 part of their body, their ramifica- 

 tions proceeding from the two ovi- 

 ducts (&, &), which have a dilata- 

 tion (I) at their point of junction. 

 There is much obscurity about the 

 history of the embryonic develop- 

 ment of these animals; and the 

 facts observed by Siebold seem to 

 be best explained upon the hypo- 

 thesis, that what has been usually 

 considered as an egg is really an 

 egg- capsule containing several em- 

 bryoes with a store of supplemen- 

 tal yolk, as in Purpura ( 351), 

 which yolk is swallowed by the 

 embryoes at a very early period ol 

 their development within the cap- 

 sule. After their emersion from 

 the capsule, the embryoes bear so 

 strong a resemblance to certain 

 Infusoria, as to have led Prof. 

 Agassiz to the conclusion, that the 

 genera Paramecium and Kolpoda 

 are nothing else than Planarian 

 larvse ( 266, note). This point, 

 however, is still a matter for inves- 

 tigation. 1 The Planariee, however, 

 do not multiply by eggs alone; for 

 they occasionally undergo sponta- 

 neous fission in a transverse di- 

 rection, each segment becoming a 

 perfect animal; and an artificial 

 division into two or even more 

 parts may be practised with a like 

 result. In fact, the power of the Planarise to reproduce portions 



1 See 129 of Siebold and Stannius's " Vergleichende Anatomic ;" also " Muller's 

 Archiv," 1850, p. 485. 



Structure of Polycelis levigatus (a Plana- 

 rian worm) : a, mouth, surrounded by its 

 circular sucker; 6, buccal cavity; c, ceso- 

 phageal orifice; d. stomach; e, ramifica- 

 tions of gastric canals; /, cephalic ganglia 

 and their nervous filaments; g, g, testes; h, 

 vesicula seminalis; t, male genital canal; 

 k, k, oviducts; I, dilatation at their point of 

 junction; m, female genital orifice. 



