General account

The egg is essentially the only developmental stage (except a coracidium in certain orders) which is exposed to the external environment. A number of morphological and physiological adaptations allow the egg to survive in this hazardous environment – and at the same time respond sensitively to those stimuli which enable it to hatch in the appropriate place in the appropriate host.

Eggs can be divided into two groups: group I (p. 170), those from cestodes whose hosts have aquatic associations; and group II (p. 170), those whose hosts have terrestrial associations. The protective membranes developed in these two groups tend to be strikingly different. In group I, for example, the pseudophyllidean egg has a sclerotin (p. 172) capsule (Fig. 7.1(6)) and a ciliated embryophore, whereas in group II, the cyclophyllidean egg usually (Fig. 7.1 (a)) has a thick keratin embryophore and a thin capsule. In the Taeniidae the embryophore is made of closely fitting keratin blocks (Fig. 7.1(c)) which separate under the action of gut enzymes and allow the oncosphere to hatch (p. 191).

The egg and reproductive system thus present an elegant model for the study of a number of areas of physiological interest, especially the ultrastructure and cytochemistry of the spermatozoa, ova and embryonic envelopes (see p. 166), as well as the physiological processes involved in egg hatching and the subsequent ‘activation’ of the released oncosphere.