944 WORMS 



had been previously ranked as a distinct group. These are not 

 found, like the preceding, in the cavity of the alimentary canal of 

 the animals they infest, but always occur in the substance of solid 

 organs, such as the glands, muscles, &c. They present themselves to 

 the eye as bags or vesicles of various sizes, sometimes occurring 

 singly, sometimes in groups ; but upon careful examination each 

 vesicle is found to bear upon some part a ' head ' furnished with 

 booklets and suckers ; and this may be either single, as in Cysticercus 

 (the entozoon whose presence gives to pork what is known as the 

 1 measly' disorder), or multiple, as in Ccenurus, which is developed in 

 the brain, chiefly of sheep, where it gives rise to the disorder known 

 as * the staggers.' Xow, in none of these cystic forms has any 

 generative apparatus ever been discovered, and hence they are ob- 

 viously to be considered as imperfect animals. The close resemblance 

 between the ' heads ' of certain Cysticerci and that of certain Tcenice 

 first suggested that the two might be different states of the same 

 animal ; and experiments made by those who have devoted them- 

 selves to the working out of this curious subject have led to the 

 assured conclusion that the cystic Entozoa are nothing else than 

 cestoid worms, whose development has been modified by the 

 peculiarity of their position, the large bag being formed by a sort 

 of dropsical accumulation of fluid when the young are evolved in the 

 midst of solid tissues ; whilst the very same bodies, conveyed into the 

 alimentary canal of some carnivorous animal which has fed upon the 

 flesh infested with them, begin to bud forth the generative segments, 

 the long succession of which, united end to end, gives to the entire 

 series a band-like aspect. 



Other forms of Entozoa belong to the Nematoid or thread-like 

 order of which the common Ascaris may be taken as a type ; one 

 species of this (the A . lumbricoides or ' round worm ') is a common 

 parasite in the small intestine of man, while another (the Oxyuris 

 vermicularis or * thread-worm ') is found rather in the lower bowel 

 and they are much less profoundly degraded in their organisation ; 

 they have a distinct alimentary canal, which commences with a mouth 

 at the anterior extremity of the body, and which terminates by an anal 

 orifice near the other extremity ; and they also possess a regular 

 arrangement of circular and longitudinal muscular fibres by which 

 the body can be shortened, elongated, or bent in any direction. The 

 smaller Nematode worms, by some or other of which almost every 

 vertebrated animal is infested, are so transparent that every part of 

 their internal organisation may be made out, especially with the 

 assistance of the compressor, without any dissection ; and the study 

 of the structure and actions of their generative apparatus has yielded 

 many very interesting results, especially in regard to the first forma- 

 tion of the ova, the mode of their fertilisation, and the history of 

 their subsequent development. 1 Some of the worms belonging to 

 this group are not parasitic in the bodies of other animals, but live 

 in the midst of dead or decomposing vegetable matter. Others, such 

 as Gordius or the * hair-worm,' are parasitic for the greater part of 



1 See particularly the various recent memoirs of Van Beneden and of Boveri, based 

 on a study of Ascaris megalocephala. 



