. Animal biology. Zoology; Biology. PHYLUM PLATYHELMINTHES 171 /?oste//un7 Hooks ?Sucker-. excretion, and elimination are still carried on in the same manner as in the nonparasitic forms. Anal openings have been described in some trematodes and openings from the branches of the gastrovasciilar ca\'ity to the outside in turbellarians, but the extent to which these can function in egestion is not known. 198. Reproduction.—Reproduction in this phylum occurs both sexually and asexually. Sexual reproduction is, however, the more usual type. Asexual reproduction in a planarian is usually by transver
. Animal biology. Zoology; Biology. PHYLUM PLATYHELMINTHES 171 /?oste//un7 Hooks ?Sucker-. excretion, and elimination are still carried on in the same manner as in the nonparasitic forms. Anal openings have been described in some trematodes and openings from the branches of the gastrovasciilar ca\'ity to the outside in turbellarians, but the extent to which these can function in egestion is not known. 198. Reproduction.—Reproduction in this phylum occurs both sexually and asexually. Sexual reproduction is, however, the more usual type. Asexual reproduction in a planarian is usually by transverse fission, but a type of frag- mentation has also been described in which the body breaks up into a number of fragments each of which by a process analogous to regeneration becomes a complete indiiddual. The production of proglottids is also asexual reproduction. 199. Occurrence and Economic Importance.— The phylum Platyhelminthes contains a large number of species, and a very large percentage of vertebrates is infected by the parasitic forms. From an economic standpoint the free-living flat- worms are of no importance, but both trema- todes and cestodes produce a great deal of injury to domestic animals and to man. Among trema- todes several flukes are parasitic in man. The . , , most serious of these are the blood flukes (genus the two sides are not Schistosoma). Two species of blood flukes occur ±°^''^ i^ true perspective. „ . , _-. T T They should be shown in m man m Africa, one m the West indies, and exact side view. another is common in parts of Japan and China. Human tapeworms may cause serious symptoms, but usually do not, and are rarely dangerous to life. Nevertheless the larvae of one tapeworm, Echinococcus granulosus (Batsch), which lives as an adult in the dog, may occur in man, where they form cysts known as hydatids and, if not removed by operation, are often fatal. The injuries to domestic animals caused by tapeworms, though not so serious as to cause th
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