THE PHYSIOLOGY OF TAPEWORMS

Summary 1. Cestodes have a high carbohydrate content in the form of glycogen which is stored mainly in the parenchyma. The proportion of ether‐extractable material is also high and consists mainly of phospholipids. The protein content–probably mainly scleroproteins–is unusually low, being less than the sum of the carbohydrate and fat contents. 2. The following physico‐chemical properties of cestodes are known: ( a ) the osmotic pressure of the parasites is in general lower than that of the surrounding host tissue or fluid; ( b ) the cuticle is freely permeable to water; (c) the internal fluid of cysticercoids is merely a transudate of the host serum; ( d ) cestodes have a p H tolerance range of approximately 4–11. 3. Owing to the failure to establish aseptic conditions attempts to cultivate cestodes outside the host body have until lately been unsuccessful. Recent work has shown that once asepsis is obtained it is possible to cultivate plerocercoid larvae in vitro for periods up to 10 months at room temperatures. By raising the temperature of cultivation to 40 0 C, larvae can be induced to become sexually mature strobilae in vitro . This technique should allow of considerable advances in our knowledge of cestode physiology. 4. The evidence suggests that immunity to digestion of cyclophyllidean cestodes from the host digestive juices is due to the protective action of the cuticle, and that anti‐enzymes are not produced. Adult worms are quite resistant to alkaline pancreatic juice in which they are normally bathed. Digestion of the strobila can, however, be obtained by subjecting the cuticle to both acid and alkaline baths previous to enzymatic action; this effect may be due to the interaction of these substances at the cuticle surface, opening up the surface and allowing the enzymes to act. When the eggs are eaten by the intermediate host, the embryophore surrounding the onchosphere will only dissolve after immersion in dilute acid followed by alkali; consequently, the onchosphere can only be freed in the intestine and not in the stomach. When u cysticercus is swallowed by the final host similar factors operate. The scolex of the cysticercus is invaginated into a ‘sleeve’ which protects it completely from the action of the acid gastric juice. On reaching the duodenum, the presence of bile stimulates the scolex to evaginate, and the bladder (which receives both acid gastric and alkaline pancreatic juice) is digested, whereas the scolex which has received the latter only, remains undigested and elongates into a typical strobila. 5. Knowledge of metabolism is scanty. It is known that tapeworms can utilize glucose in quantity; and that the excess glucose is stored in the parenchyma in the form of glycogen. Plerocercoid larvae of pseudophyllidean cestodes can, however, undergo complete maturation in vitro in glucose‐free media and it is supposed that cestodes may be capable of utilizing complex carbohydrate‐protein compounds. The metabolic rate in warm‐blooded hosts is high and the glycogen reserve falls rapidly to about one‐eleventh of its initial level after 20 hr. of host starvation. The glycogen is probably decomposed anaerobically with fatty acids as by‐products; cestodes can, however, utilize oxygen if it is offered to them. 6. There is as yet no evidence to indicate that cestodes have any fat requirements. 7. Little is known regarding protein metabolism; absence of protein in the host diet has no apparent effect on the metabolism of the worm. 8. The nutritional requirements may not be as simple as is generally believed, and there is some evidence that vitamins–notably vitamin G–may be essential for normal cestode development.