ANNULOSA, OK WORMS. 



195 



FIG. 406. 



of the frequency with which the worms of the Planarian tribe present 

 themselves among collections both of marine and of fresh-water animals 

 (particular species inhabiting either locality), and on account of the curi- 

 ous organization which many of these possess. Most of the members of 

 this tribe have elongated flattened bodies, and move by a sort of gliding 

 or crawling action over the surfaces of aquatic Plants and Animals. Some 

 of the smaller kind are sufficiently transparent to allow of their internal 

 structure being seen by transmitted light, especially when they are slightly 

 compressed; and the accompanying figure 

 (Fig. 406) displays the general conforma- 

 tion of their principal organs, as thus 

 shown. The body has the flattened sole- 

 like shape of the Trematode Entozoa; its 

 mouth, which Avhich is situated at a consi- 

 derable distance from the anterior ex- 

 tremity of the body, is surrounded by a 

 circular sucker that is applied to the living 

 surface from which the animal draws its 

 nutriment; and the buccal cavity (b) opens 

 into a short oesophagus* (c), which leads at 

 once to the cavity of the stomach. In the 

 true PlanaricB the mouth is furnished with 

 a sort of long funnel-shaped proboscis; and 

 this, even when detached from the body, 

 continues to swallow anything presented 

 to it. The cavity of the stomach does not 

 give origin to any intestinal tube, nor is it 



Erovided with any second orifice; but a 

 irge number of ramifying canals are pro- 

 longed 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 fluid ; and 

 the two principal trunks, with connecting 

 and ramifying branches, which may be 

 observed in them, are probably to be re- 

 garded 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; 



tllOUgn the COngreSS OI tWO, each impreg- ach; e, ramifications of gastric canals; 

 nnHno- DTP nva nf HIP nihpv cppma fn hp /< cephalic ganglia and their nervous 



.61, S( ms IO 06 fii amen ts; g, g, testes;/i, vesicula semi- 



generally necessary. The ovaria, as in the naiis; *, male genital canal; fc, k ovi- 



TT< i. -i J i-\ L ducts; I, dilatation at their point of 



-hntozoa, extend through a large part Of junction; m, female genital orifice. 



the body, their ramifications proceed- 

 ing from the two oviducts (k, &), which have a dilatation (I) at their 

 point of junction. There is still much obscurity about the history of 

 the embryonic development of these animals; as the accounts given 

 of it by different observers by no means harmonize with each other. 1 

 The Planariae, however, do not multiply by eggs alone; for they occa- 

 sionally undergo spontaneous fission in a transverse direction, each seg- 

 ment becoming a perfect animal; and an artificial division into two or even 



Structure of Polycelis levigatus (a 

 Planarian worm). a, Mouth surround- 



1 See Balfour's "Comparative Embryology," Vol. i., pp. 159-162. 



