On anatomical grounds the affinity of the Cestoda to the Trematoda 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 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 segmentation it fills up nearly the whole egg-shell. As was first shewn by Kölliker 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 formation of the embryo, a ciliated larva is developed (Bothriocephalus latus, ditremus, Schistocephalus dimorphus, Ligula simplicissima, etc.[95]). [95] Vide for list of such forms at present known Willemoes Suhm, No. 231. 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.