PLATYELMINTHES. 211 



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 consist- 

 ency, and is provided 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. 218, 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 segmen- 

 tation 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 segmen- 

 tation 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 formation of the embryo, a ciliated larva is developed (Bo- 

 thriocephalus 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. 



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



142 



