124 INVERTEBRATE ANIMALS. 



l)emispheres, and which spends a portion of its existence buried in 

 the celkilar tissue which separates the muscles. It may attain a 

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

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

 worm " or Trii-hina, which attacks man, pigs, &c., and exists in two 

 conditions. In one stage it is found buried 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 

 Trii-hince have no reproductive organs and are comparatively harm- 

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

 another suitable animal, the little Triddna escape from their cysts 

 and develop 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 the 

 muscles, where they surround themselves each with 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 disturbance. 



SECTION C. ROTIFERA. 



The Rotifera, or " Wheel-animalcules," 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) whicli, when in motion, vibrate 

 so rapidly as to produce the illusory impression of a quickly rotating 

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

 itants of fresh water. They are all microscopic in size, none attain- 

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

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

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

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

 finally, a well-developed water-vascular system. 



Most of the Rotifera are free-swimming active little animals (fig. 

 79, A), but some are permanently fixed, as in J/elicerta (fig. 79, B) 

 or in the Crown-animalcule {Stephanoneros). They are usually 

 simple, but they are sometimes composite, forming colonies. As a 

 rule, the male and female Rotifera differ greatly from one another, 

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

 or digestive ap]iai atus, and more or less closely resembling the young 

 forms of the species. The males, in fact, merely lead a transient 

 existence, and die as soon as they have succeeded in fertilising the 

 females. The body in most cases is very distinctly ringed or annu- 

 lated (fig. 79, A), but is not composed of distinct rings separated by 



