284 THE ANIMAL PARASITES OK MAN 



(2) By accumulation of fluid in the interior this becomes a closed sac with a wall 

 formed of a single layer of cells, forming the blastosphere or blastula. 



(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 &planula. 



(5) This becomes fixed to a rock, an invagination forms at one end, a depression 

 the stomodaeum communicating with the enteric cavity. Tentacles grow out 

 and we have a Scyphozoon 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 Ephyrula. 



(8) These develop finally into adult sexual jelly-fish, Scyphozoa, so that com- 

 paring a tapeworm with this we have (a) egg, () 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. (i) 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 zooids 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 through- 

 out 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 multiplica- 

 tion occurs at two points of the whole cycle of development, which is as follows : 

 (i) 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. 



ANATOMY OF THE CESTODA. 



If we except the tapeworms with only one proglottis, the 

 CESTOIDEA MONOZOA, Lang = Cestodaria, Monticelli, we can always 

 distinguish in the Cestodes, in the narrower sense, one scolex or head 

 and a large or small number of segments (proglottids). The SCOLEX 

 serves the entire tapeworm for fastening it to the internal surface of the 



