622 CYSTIC AND NEMATOID ENTOZOA. 



the ' Water- Vascular system,' whose simplest condition has been 

 noticed in the Wheel-Animalcule (§ 359). — Few among the recent 

 results of Microscopic inquiry have been more curious, than the 

 elucidation of the real nature of the bodies formerly denominated 

 Cystic Entozoa, which have always ranked until recently as a dis- 

 tinct 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 

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

 in the brain, chiefly of Sheep, giving rise to the disorder known as 

 'the Staggers.' Now in none of these Cystic forms has any Gene- 

 rative apparatus ever been discovered, and hence they are obviously 

 to be considered as imperfect animals. The close resemblance 

 between the ' Head' 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 

 themselves 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 Gene- 

 rative segments, the long succession of which, united end-to-end, 

 gives to the entire series a Worm-like aspect. 



485. The higher forms of Entozoa, belonging to the Nematoid 

 or thread-like Order, — of which the common Ascaris may be taken 

 as a type, one species of it (the A . lumbricoid.es, or ' Round Worm ') 

 being a common parasite in the Small Intestine of Man, while 

 another (the A . vermicularis, or ' Thread Worm ') is found rather 

 in the lower bowel, — approach more closely to the ordinary type 

 of conformation of Worms ; having a distinct Alimentary Canal, 

 which commences with a mouth at the anterior extremity of the 

 body, and which tei'ininates by an anal orifice near the other 

 extremity ; and also possessing a regular arrangement of circular 

 and longitudinal Muscular fibres, by which the body can be 

 shortened, elongated, or bent in any direction. The smaller 

 species of Ascaris, by some or other of which almost every Verte- 

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

 their internal organization may be made-out, especially with the 

 assistance of the Compressor, without any dissection ; and the 



