121 INVEKTLj 



hemispheres, <anJ whicli spends a portion fif its existence buried in 

 the celhilar tissue «vliieh separates the muscles. It may attain a 

 length (if several feet, and has an extraordinary but still incom- 

 pletely understood history. Equally famous is the little " Muscle- 

 worm " or Tricliina, which attacks man, pigs, &c., and exists in two 

 conditions. In one stage it is found liuried in vast numbers in the 

 muscles, each little worm — a thirtieth of an inch or less in length — 

 being enclosed in a little whitish capsule. When thus encysted, the 

 Trichituf have no leproductive organs and are comparatively harm- 

 less. If, however, a, jiortion of the infected muscle be eaten by 

 another suitalile animal, tlie little Ti-ii-hiiup escape from their cj-sts 

 and develoji reproductive organs. They then produce young, in vast 

 numbers, within the intestine of their new host ; and the young bore 

 their way through the walls of the intestine, and travel into tlie 

 muscles, A\'liere tliey sun'ouml themselves each witli a little capsule, 

 and become quiescent. When this stage has been reached, the 

 animal attacked is in safety; but the migration of the worms from 

 the intestine into the muscles usually gives rise to extremely serious 

 or even fatal constitutional disturliance. 



SECTION 0. ROTIFEllA. 



The liiiiij'eni, or " Wheel-a,nimakades," derive their popular name 

 from the fact that the anterior end of the body is furnished with 

 one or two circlets of cilia (fig. 79) which, when in motion, vibrate 

 so rapidly as to ])roduce the illusory impression of a quickly rotating 

 toothed wheel. The Rotifera are all aquatic, and are mostly inhab- 

 it.irits of fresh water. They are all nncroscopic in size, none attain- 

 ing a greater length than l-36th of an inch. In the females there 

 is a ilistinct mouth, intestinal canal, and anus. A nervous system 

 is also jiresent, consisting of ganglia placed near the anterior ex- 

 tremity of the body and sending filaments backwards. There is, 

 hnally, a well-developed water-vascular system. 



Most of tlie liDtlfiTii, are free-swimming active little animals (fig. 

 7!), A), but .some ai'e permanently fixed, as in McUi-erta (fig. 71), T>) 

 or in the C'rowu-.-iniinalcule {Sti-pldtiiiiri'ni.i). They are u.sually 

 simjile, but they are sometimes coiu])r)site, forming colonies. As a 

 rule, the )iiale and female Ilotifrrn differ greatly from one another, 

 the nudes being smaller than the females, devoid of any masticatory 

 or digestive appar.-itiis, and more or less closely resendjiing the young 

 foians of the sjiecies. The mali's, in fact, merely lead a transient 

 I'vistcucc, .'ind die as soon as they have s\icceeded in fertilising the 

 IciM.ih-s. The body in most cases is very distinctly ringed or annu- 

 lalcij fll- 7i», A), l)iit is not eomposeil of distinct rings separated by 



