ADAPTATIONS TO PARASITIC EXISTENCE

I. SPECIALIZATION

A. Certain sensory organs are highly developed,

1. Chemosensory organs among cercariae for the detection of snail mucus.

2. Thermosensory organs among hookworm larvae for the detection of homeothermic animals.

B. Resistant stage (e.g. cyst or egg) for transferral to new hosts.

1.Cyst form of intestinal protists and helminth ova are metabolically inactive, but can survive reduced temperatures and increased osmotic pressures of freshwater.

II. LOSS OF SOME GENES AND ORGANS

A. "Unnecessary" organs become vestigial or disappear.

1. Most internal helminths lack pigments and eyes.

2. Cestodes do not have any digestive system -- nutrients are absorbed directly through the tegument.

B. Loss of capacity to synthesize enzymes, nutrients

1. Trematodes lack the ability to synthesize certain fatty acids which must be in their diet, and this "flaw" lends itself to a commensal or parasitic existence.

III. HIGH BIOTIC POTENTIAL, facilitated by

A. Hermaphroditism: Adults have functional male and female reproductive systems. In parasitic organisms, this hermaphroditism is simultaneous, as opposed to sequential.

1. Characteristic of Platyhelminthes, including cestodes (tapeworms) and trematodes (flukes)

2. Cross-fertilization occurs across proglottids of adjacent cestodes or even the same cestode, and across adjacent flukes.

B. Parthenogenesis: Development of an egg without fertilization by a sperm cell. Common among insects, e.g. aphids.

C. Polyembryony: Larval forms undergo a form of budding in the intermediate host. This is observed in digenetic trematodes (flukes) and some wasps.

C. Strobilization: Segments (proglottids) develop behind a holdfast organ (scolex). Each proglottid develops within it complete male and female reproductive systems. Following self- or cross-fertilization, the male reproductive structures deteriorate and eggs mature mature. Proglottids which are furthest away from the scolex are gravid, and will release eggs when the proglottid is shed.

Adult Echinococcus granulosus, the small dog tapeworm, showing the anterior scolex with adhesive suckers, followed by a strobila consisting of one immature, one mature, and one gravid proglottid.