Structure and Development of Echinococcus (Hydatid).
Fig. 251.—Echinococcus veterinorum: the fibrous sac enclosing the echinococcus has been opened and laid back in five parts, so that the surface of the bladder worm may be seen, with the brood capsules, visible to the naked eye, showing through it. Natural size. (After Leuckart.)
An echinococcus is a spherical or roundish bladder full of a watery liquid, which originates by liquefaction of the oncosphere, and in man may attain the size of a child’s head, but remains smaller in cattle (the size of an orange or apple). The thin wall of the bladder is composed of an external laminated cuticle (ectocyst) and an internal germinal or parenchymatous layer (endocyst). The latter again exhibits two layers: an outer layer of small cells that are less sharply defined, and an inner layer of larger cells. It contains, in addition, calcareous corpuscles, muscular fibres and excretory vessels. It is rich in glycogen.
Fig. 252.
Figs. 252 and 252A.—Diagrams of mode of formation of brood capsule and scolices. (1) Wall of mother cyst, consisting of ectocyst and endocyst; (2) theoretical stage of invagination of wall; (3) a brood capsule with the layers of the wall in the reverse position to that in the mother cyst; (4) evagination of wall; (5) invagination; (6) fusion to form the solid scolex; (7) invagination of fore-part of scolex into hind-part. (Note.—The size of the scolex is much out of proportion to the brood capsule.) (Stephens.)