(3) One end of the sac is invaginated, forming a gastrula.
(4) The gastrula pore or mouth closes, forming again a sac, the walls of which have two layers, forming a planula.
(5) This becomes fixed to a rock, an invagination forms at one end, a depression—the stomodæum—communicating with the enteric cavity. Tentacles grow out and we have a Scyphozoön polype, Scyphistoma or Scyphula. It is to this stage that Steenstrup gave the name “nurse” (“wet-nurse”), because it nourished or produced asexually the succeeding forms.
(6) Asexual reproduction by transverse fission occurs in this, forming a pile of saucer- or pine-cone-like animals which before this time had been considered to be a distinct animal, which was called strobila from its resemblance to a pine-cone. This is the alternate generation.
(7) The individuals of the strobila become free and are called Ephyrulæ.
(8) These develop finally into adult sexual jelly-fish, Scyphozoa, so that comparing a tapeworm with this we have (a) egg, (b) scolex (= Scyphula or “nurse”), (c) asexual reproduction of the tapeworm chain (= strobila), (d) development of the individuals of the chain (proglottids) into sexual adults.
Van Beneden’s terminology for these stages is the following: Ciliated embryo = protoscolex; scyphistoma = deutoscolex (or scolex); free Ephyrula = proglottis. According to this view, as is the case in many endoparasitic Trematodes, asexual reproduction by budding occurs at two stages of the whole cycle of development, viz. (1) in the formation of the scolex by budding from the bladder (“nurse”), (2) in the formation of the strobila by budding from the scolex (“nurse”).
But in cysticercal larval forms it appears that the scolex does not arise in this way but is simply a part of the proscolex (hexacanth embryo), becoming invaginated into it for protection, so that there is no asexual gemmation here. It has been questioned also whether the strobila also arises by gemmation. If it does, the tapeworm is a colony of zoöids produced by budding from the asexual scolex; if it is not produced in this way, then the tapeworm is to be regarded as an individual in which growth is accompanied by segmentation. Against the “colony” view are the facts that the muscular, nervous, and excretory systems are continuous throughout the worm, and that some tapeworms, such as Ligula, are unsegmented.
Finally, if the tapeworm is an individual the question arises which is the head end. As new segments are formed at the neck, and as this point in annelids is the antepenultimate segment, the scolex must be the last or posterior segment. The caudal vesicle or bladder of larval forms is consequently anterior. According to this view, in tapeworms as among many endoparasitic flukes, an asexual multiplication occurs at two points of the whole cycle of development, which is as follows: (1) egg, (2) oncosphere or hexacanth embryo, (3) bladder (cysticercus or hydatid), (4) (after digestion of the bladder) by budding, the scolex, (5) by budding from the scolex the sexual proglottids, (6) the egg; (4) and (5) being the two asexual stages.