174 CESTODA. 



the parent. It seems probable that the second and third generations are 

 produced asexually, the generative organs not being developed; while the 

 young Gyrodactylus of the first generation springs from a fertilized ovum 

 (Wagener, No. 214). 



CESTODA. 



On anatomical grounds the affinity of the Cestoda to the Trema- 

 toda has been insisted on by the majority of anatomists. The 

 existence of such intermediate forms as Amphilina tends to strengthen 

 this view ; and the striking resemblances between the two groups in 

 the structure of the egg and characters of the metamorphosis appear 

 to me to remove all doubt about the matter. 



The ripe egg is formed of a minute germ enveloped in yolk cells, 

 the whole being surrounded by a membrane, which is very delicate in 

 most forms, but in certain types has a firmer consistency, and is pro- 

 vided with an aperture, covered by an operculum, by which the larva 

 escapes. 



The early development, up to the formation of a six-hooked larva, 

 generally takes place in the uterus, but in the types with a firmer 

 egg-shell it takes place after the egg has been deposited in water. 



The segmentation (E. van Beneden, No. 2 1 8, Metschnikoff, No. 228) 

 is complete, and during its occurrence the yolk cells surrounding the 

 germ are gradually absorbed, so that the mass of segmentation spheres 

 grows in size, till at the close of segmentation it fills up nearly the 

 whole egg-shell. 



As was first shewn by Kolliker for Bothriocephalus salmonis, 

 the embryonic cells separate themselves at the close of segmentation 

 into a superficial layer and a central mass. 



The further development takes place on two types. In the cases 

 where the egg-shell is strong, and the egg is laid prior to the forma- 

 tion of the embryo, a ciliated larva is developed (Bothriocephalus 

 latus, ditremus, Schistocephalus dimorphus, Ligula simplicissima, 

 etc. 1 ). 



Of these forms Bothriocephalus latus may be taken as type. 



The development of the embryo requires many months for its 

 completion. The outer layer becomes ciliated while the central mass 

 has already become developed into a six-hooked embryo. The 

 embryo leaves its shell by the opercular aperture, and for some 

 time swims rapidly about by means of its long cilia. The ciliated 

 coating is eventually stripped off, and the six-hooked larva emerges. 



In the second type of embryo the external cellular layer does 

 not become ciliated. This is the most usual arrangement, and is even 

 found in many species of Bothriocephalus. 



The central mass of cells becomes developed, as in the other type, 

 into a six-hooked (rarely four-hooked) embryo (fig. 96 G), but the 

 superficial layer separates from the central, and either disappears or 



1 Vide for list of such forms at present known Willemoes Sulim, No. 231. 



