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 the death of the 

 animal, are often sufficient to interfere somewhat with their usefulness. 



Fig. 84. — Scolex of the 

 pork tapeworm, Taenia 

 solium Linnaeus. {From 

 Leuckart, " Parasiten des 

 Menschen.") X30. 

 Shows a rostellum with 

 hooks, this being an armed 

 tapeworm. The illustra- 

 tion is not artistically cor- 



