CCENURUS CEREBRALIS. 



129 



Fig. 64 



(336.) The ENTOZOA, as the name implies, are nourished within the 

 bodies of other animals, from the juices of which they derive their 

 sustenance. It may naturally be supposed that, living under such cir- 

 cumstances deprived of all power of locomotion, debarred from the in^ 

 fluences of light, and absolutely dependent upon the fluids wherein they 

 are immersed for nutriment the Entozoa have little occasion for that 

 elaborate organization needful to animals living in immediate communi- 

 cation with external objects. 



(337.) We find, therefore, among these creatures, certain races whose 

 structure is of the simplest character possible, in adaptation to the 

 circumscribed powers of which they are capable. Yet, however ap- 

 parently insignificant some of them may appear, they not unfrequently 

 become seriously prejudicial to the animals wherein they are found, by 

 the prodigious numbers in which they exist, or from their growth in 

 those organs more especially essential to life ; and 

 not a few of them, from their dimensions alone, 

 sometimes prove fatal, as may be supposed from 

 a mere inspection of the annexed figure (fig. 64), 

 representing an Entozoon developed in the ab- 

 dominal cavity of a fish. 



(338.) There are probably no races of animals 

 which are not infested with one or more species 

 of these parasites, from the microscopic infusoria 

 up to man himself ; and sometimes several dif- 

 ferent forms are met with in the same species, to 

 which they would appear to be peculiar ; nay, in 

 some cases the Entozoa would seem themselves 

 to enclose other species parasitically dwelling in 

 their own bodies. Neither is their existence con- 

 fined to any particular parts ; they are met with 

 in the alimentary canal, in the liver, the kidneys, 

 the brain, the arteries, the bronchial passages, the 

 muscles, the cellular tissue, and, in fact, in almost 

 all the organs of the body. 



(339.) The Cystiform Helminthozoa, generally known by the name of 

 Hydatids, are the simplest in structure ; and with these, therefore, we 

 shall commence our inquiry into the economy of these creatures. The 

 Ccenurus cerebralis (fig. 65), one of the most common, occurs in the 

 brain of sheep, and is the cause of a mortal disease but too well known 

 to the farmer ; it is likewise occasionally developed in other ruminating 

 quadrupeds, and, by partially destroying the cerebral substance, soon 

 proves fatal. This Entozoon, represented in the figure of ordinary size, 

 consists of a delicate transparent bladder, the walls of which, during 

 the life of the creature, are visibly capable of spontaneous contractions 

 on the application of stimuli. To this bladder, or common body, are 



Ligula simplicissima in 

 the abdominal cavity of a 

 Minnow. 



