ZOOGENESIS 



I. The jointed tapeworms. From a head end, or "scolex," 

 which is commonly radially symmetrical and often four-sided 

 (figs. 5, p. 5; 8i, 83, p. 161), a series of units is continually budded 

 off which, as they are shoved further and further from the scolex 

 by the budding off of new units, become more and more developed. 

 These units — the proglottides — are usually strongly flattened 

 and have a more or less well marked dorsal and ventral surface. 

 The proglottides usually differ more or less on either side of the 

 plane passing through the center of the so-called dorsal and ven- 

 tral surfaces. 



In these jointed tapeworms we see a rather close approach to 

 the type of development characteristic of various jellyfishes — 

 for instance the common Aurellia. In these jellyfishes the t^g 

 gives rise to a little creature shaped like an inverted bell attached 

 by the handle. From the margin of this bell two tentacles arise 

 opposite each other in quick succession, so that at the two ten- 

 tacle stage the little creature is bilaterally symmetrical — or 

 rather biradial. After this, other tentacles appear. Subsequent 

 to the formation of the complete circle of tentacles, the larva 

 undergoes more or less extensive reproduction by the formation 

 of buds which separate off and grow into new and independent 

 animals, and also by division into two or more parts, each part 

 after separation becoming a new animal. In this connection it 

 may be mentioned that in certain tapeworms the larval form may 

 in somewhat comparable fashion produce from its internal walls 

 one or two generations of secondary vesicles which project into 

 it, the cestode heads originating in special small brood capsules 

 on these secondary vesicles. In these cases the number of sepa- 

 rate tapeworms which arise from a single embryo is enormous. 

 In the little bell-shaped jellyfish larva, after reproduction by 

 budding and by fission have proceeded for some time the body 

 begins to divide transversely, and the tentacles disappear. The 

 body now elongates, and little plate-like or saucer-shaped bodies 

 with four pairs of marginal lobes are detached one by one from 

 the outer end. These float away and grow into jellyfishes. The 

 distal proglottides of the jointed tapeworms are also detached, 

 one by one or in groups, but in the case of the tapeworm the pro- 

 glottides are fully developed before detachment. It may be re- 

 marked that not all of the tapeworms are jointed, some being 

 single units. 



