66 Invertebrata. 



of these multiply, as, for example, the common Mermis 

 aloicans, that they have given rise to the belief that 

 they have fallen as ' worm-ra.ins.' These worms are 

 called Gordiaceae and are distinguished from the other 

 round-worms by the rudimentary condition of their 

 digestive canal. They are also remarkable for their 

 extreme tenacity of life, as they can be dried into hard 

 brittle threads and yet appear lively and active on 

 being moistened. 



CLASS IV. Acanthocephala. The ' thorn-headed ' 

 worms are rounded, or cylindrical, each with a pro- 

 trusible proboscis armed with many recurved hooks. 

 They are remarkable for the total absence of the mouth 

 and intestine in their adult condition. The common- 

 est species are found in the intestines of swine, &c., 

 with their heads buried in the substance of the wall 

 of the digestive tube. 



CHAPTER XII. 



NON-PARASITIC WORMS. 



CLASS V. Wheel- Animals, Rotatoria. On tracing 

 the development of the more complex free worms we 

 find that the larva, after emerging from the egg, ap- 

 pears as a free-moving creature, with circlets of vibrat- 

 ing cilia aiound its extremities, these ciliary lobes 

 being in some forms large and rounded (fig. 47, A). 



In rain pools and ditches, small creatures are fre- 

 quently met with which resemble the larvae of worms, 



