PLANAEIA 



947 



regarded in the light of a gastro-vascular system, the function of 

 which is not only digestive, but also circulatory. 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 gene- 

 rally necessary. The ovaria, as 

 in the Entozoa, extend through 

 a large part of the body, their 

 ramifications proceeding from 

 the two oviducts (&, &), w T hich 

 have a dilatation (I) at their 

 point of junction. The Pla- 

 narice T do not multiply by eggs 

 alone ; for they occasionally un- 

 dergo spontaneous fission in a 

 transverse direction, each seg- 

 ment 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 Planariw 

 to reproduce portions which 

 have been removed seems but 

 little inferior to that of the 

 Hydra ; a circumstance which 

 is peculiarly remarkable when 

 the much higher character of 

 their organisation is borne in 

 rnind. They possess a distinct 

 pair of nervous ganglia (/,/), 

 from which branches proceed to 

 various parts of the body ; and 

 in the neighbourhood of these 

 are usually to be observed a 

 number (varying from two to 

 forty) of ocelli or rudimentary 

 eyes, each having its refracting 

 body or crystalline lens, its pig- 



FIG. 714. Structure of Polycelis levi- 

 gatus (a Planarian worm) : a, mouth, 

 surrounded by its circular sucker; b, 

 buccal cavity ; c, oesophageal orifice ; 

 d, stomach ; e, ramifications of gastric 

 canals ; /, cephalic ganglia and their 

 nervous filaments ; g, g, testes ; h, 

 vesicula seminalis ; i, male genital 

 canal; k, k, oviducts; I, dilatation at 

 their point of junction ; m, female 

 genital orifice. 



ment-layer, its nerve-bulb, and 



its cornea-like bulging of the 



skin. The integument of many 



of these animals is furnished 



with cells containing rods or spindles which are very possibly 



comparable to the ' thread-cells ' of zoophytes. 2 



Annulata, This class includes all the higher kinds of worm -like 

 animals, the greater part of which are marine, though there is one 

 well-marked group the members of which inhabit fresh water or live 



See Balfour's Comparative Embryology, vol. i. pp. 159-162. 

 - For further information regarding the Turbellaria consult Dr. L. Graff's article 



naturelle des Turbellaries, 



Lille, 1879. On transverse fission, see Bell, Journ. Boy. Microsc. Soc. (2) vi. p 1107 



8p2 



'.MY/. 



lie, 



