. Elementary text-book of zoology. 142 PLATYHELMINTHES. its length into a large two-layered sac called the sporocyst. The inner layer buds cells into theinternal cavity, which develop into organisms called redim through a monila and gasirula stage. A redia has an elongated body with mouth at the anterior end, a pharynx and simple intestine. Externally it has a collar or thickened ridge round the anterior end, behind which is a small pore into the body-cavity, and a pair of processes towards the hind end. It also has excretory tubules. A redia when developed bursts through the brood-sac or spor
. Elementary text-book of zoology. 142 PLATYHELMINTHES. its length into a large two-layered sac called the sporocyst. The inner layer buds cells into theinternal cavity, which develop into organisms called redim through a monila and gasirula stage. A redia has an elongated body with mouth at the anterior end, a pharynx and simple intestine. Externally it has a collar or thickened ridge round the anterior end, behind which is a small pore into the body-cavity, and a pair of processes towards the hind end. It also has excretory tubules. A redia when developed bursts through the brood-sac or sporocyst and eats its way through the snail. Eventually it produces, by budding of its internal cells, a number of cercarice which are young or larval flukes. The cercaria escapes by the genital pore of the redia and out of the snail into the water. It has a rounded body and vibratile tail. Two suckers, a mouth, pharynx, and simple bilobed intestine can be distinguished, and there is also a flame-cell excretory system. The surface is dotted with cystogenous cells which produce the cyst. The cercaria works its way to the edge of the pond (the snail may be in grass already), up a blade of grass or other plant and there loses its tail, encysts and remains dormant. Should the cyst be introduced into the stomach of the sheep the cercaria escapes, passes up the bile-duct, and develops in a few weeks into a young fluke. We have to add that the sporo- cyst may produce fresh sporocysts by binary fission and that the redia may give rise to fresh generations of redise. Fig. 74.— Sporocyst containing Redise. I^ig- 75-—A Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Masterman, Arthur Thomas. Edinburgh, E. & S. Livingstone
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