There is no free-swimming planula larva, but the stage corresponding to it is passed over in an enveloping cyst, which is secreted round the embryo by its own ectodermal layer, shortly after the germ-layer formation is complete, i.e.
The spadix forms a gelatinous cyst, the so-called acrocyst (ac), external to the gonotheca (gth), enclosing and protecting the embryos.
Thus a mass or chain of embryos is produced, lying in a common cyst, and developing as their larval host develops.
The peculiar feature of the larval history of Cestodes is the development in most cases of a cyst or hydatid on the inner wall of which the scolex is formed by invagination.
The cyst is filled with a toxic fluid and may bud off new or daughter scolices.
In some genera a " urocyst " is formed, the tail of which gives rise to a new cyst and a fresh scolex.
The most remarkable feature of this cystic development is the formation in many genera of several internal buds within a common cyst, each of which forms an independent inverted scolex (Coenurus, Polycercus); or these internal vesicles may bud off a large number of scolices on their external surface (Staphylocystis).
On this view, therefore, at least two asexual generations (embryo and scolex) alternate with a sexual one (proglottides); and in the case of Staphylocystis the cyst contains two asexually produced generations, so that in such forms three stages (embryo, primary scolex-buds, secondary scolices) intervene between the proglottis of a Cestode and that of its offspring.
At m the ideal mode of origin is shown in order to illustrate the fact that the daughter cyst is comparable to the fore-body of a cysticercus.
(From Lankester's Treatise on Zoology, part iv.) purposes inside the cyst, which is itself an organ comparable to an amnion.
When that occurs, the cyst is dissolved and the minute fluke works its way down the alimentary canal into some part of which it inserts its suckers and commences to feed on the blood of its host.
The cyst should be treated like a liver-abscess, by incision through the abdominal or thoracic wall, by circumferential suturing and by exploration and drainage.
Single cyst germinating (X200).
Deformator gives rise to a "gall" on the arm of the host, one joint of the pinnule growing round the worm so as to enclose it in a cyst (see fig.
(After Beard.) E, Portion of the arm of Pentacrinus, showing a cyst containing Myzostoma.