ENTOZOA ; STERELMINTHA. 315 



with an orifice at each end, floating freely in an abdominal cavity, 

 as in higher animals ; and there are also traces of a nervous and 

 muscular system, more or less developed. This division evi- 

 dently approximates to the Annelida. In the lower division, 

 CCELELMINTHA, there is no distinct alimentary canal ; the 

 cavities for the reception of food, as well as those for other pur- 

 poses, being, as it were, channelled out of the soft, almost homo- 

 geneous, tissues of the body. Some of these, too, preserve the 

 Worm-like form ; but in others this can scarcely be traced. 



853. SECTION I. STERELMINTHA. A characteristic ex- 

 ample of this group, possessing a very distinct nervous system, is 



presented by the 

 Linguatula tce- 

 nioides(Fig. 534); 



FIG. 534,-LiNWATULA TVEmoiDEs. a W rm which 



infests the frontal 



sinuses of the horse and dog. The Ascaris lumbricoides, or 

 Round Worm of the human intestines, also belongs to this group. 

 It infests not only man, but many of the lower animals ; and 

 sometimes occasions severe disease, and even death. It derives 

 its second or specific name from its resemblance to the Earth- 

 worm. The short active Thread-worms, sometimes infesting 

 the lower part of the intestine, and giving rise to great irrita- 

 tion, but not to such severe consequences as the preceding, con- 

 stitute another species of the same genus. We may also notice 

 the Filaria, or Guinea-worm, which burrows in the flesh of 

 man and other animals in warm climates ; if undisturbed, it will 

 often continue its operations for a considerable time without 

 much uneasiness ; but if disturbed, it sometimes occasions the 

 most excruciating pain. When it shows itself externally, it 

 must be extracted very slowly and carefully, for fear of breaking 

 it ; since, if this should occur, the part remaining in the body 

 would retreat and continue to exist in its previous situation, 

 re-forming the remainder. This Worm grows to the length of 

 several yards. 



854. SECTION II. CCELELMINTHA. A characteristic example of 

 this group, in which the worm-like form is preserved, with a 

 considerable degree of complexity of organisation, is the Tcenia 



