2l8 CESTODA. 



Some important variations of the typical development are known. 



The so-called head or scolex may be formed without the intervention 

 of a cystic stage. In Archigetes (Leuckart, No. 227), which infests, in 

 the Cysticercus condition, the body-cavity of various invertebrate forms 

 (Tubifex, etc.), the six-hooked embryo becomes elongated and divided into 

 two sections, one forming the head, while the other, with the six embryonic 

 hooks, forms an appendage, homologous with the caudal vesicle of other 

 Cysticerci. 



The embryo of Tcenia elliptica similarly gives rise to a Cysticercus 

 infesting the dog-louse (Trichodectes cants], without passing through a 

 vesicular condition ; but the caudal vesicle disappears, so that it forms 

 simply a scolex. These cases may, it appears to me, be probably regarded 

 as more primitive than the ordinary ones, where the cystic condition has 

 become exaggerated as an effect of a parasitic life. 



In some cases the larva of a Taenia has a free existence in the scolex 

 condition. Such a form, the larva of Phyllobothrium, has been observed by 

 Claparede 1 . It was not ciliated, and was without a caudal vesicle; and 

 was no doubt actively migrating from an intermediate host to its permanent 

 host. 



Scolex forms, without a caudal vesicle, are found in the mantle cavity 

 of Cephalopoda, and appear to be occupying an intermediate host in their 

 passage from the host of the cystic worm to that of the sexual form. 



Archigetes, already mentioned, has been shewn by Leuckart (No. 227) 

 to become sexually mature in the Cysticercus state, and thus affords an 

 interesting example of paedogenesis. It is not known for certain whether 

 under normal circumstances it reaches the mature state in another host. 



Amphilina. The early stages of this interesting form have been 

 investigated by Salensky (No. 229), and exhibit clear affinities to those of 

 the true Cestoda. An embryonic provisional skin is formed as in Cestodes ; 

 and pole-cells also appear. Within the provisional skin is formed an 

 embryo with ten hooks. After hatching the provisional skin is at once 

 thrown off, and the larva, which is then covered by a layer of very 

 fine cilia, becomes free. The further metamorphosis is not known. 



BIBLIOGRAPHY. 

 Turbellaria. 



(181) Alex. Agassiz. "On the young stages of a few Annelids" (Planaria 

 angulatd). Annals Lyceum Nat. Hist, of Nav York, Vol. vili. 1866. 



(182) Dalyell. " Powers of the Creator. " 



(183) C. Girard. " Embryonic development of Planocera elliptica." Jour, of 

 Acad. of Nat. Set. Philadelphia. New Series, Vol. 1 1. 1854. 



(184) Alex. Gotte. "Zur Entwicklungsgeschichte d. Seeplanarien. " Zoolo- 

 gischtr Anzeiger, No. 4, 1878. 



1 Beobachtungen iib. Anat. H. lint-wick. Wirbell. Thiere. Leipzig, 1863. 



