The venous system consists of the usual anterior and posterior cardinal veins which unite on each side into a ductus Cuvieri, and of a great subintestinal vessel of the same nature as that in embryo Elasmobranchs, which persists however in the adult. It breaks up into capillaries in the liver, and constitutes therefore the portal vein. From the liver the blood is brought by the hepatic vein into the sinus venosus. In addition to these vessels there is a remarkable unpaired sub-branchial vein, which brings back the blood directly to the heart from the ventral part of the branchial region. Metamorphosis. The larva just described does not grow directly into the adult, but first becomes a larval form, known as Ammocoetes, which was supposed to be a distinct species till Aug. Müller (No. 80) made the brilliant discovery of its nature. The Ammocoetes does not differ to any marked extent from the larva just described. The histological elements become more differentiated, and a few organs reach a fuller development. The branchial skeleton becomes more developed, and capsules for the olfactory sack and auditory sacks are established. The olfactory sack is nearly divided into two by a ventral septum. The eye (fig. 48) is much more fully developed, but lies a long way below the surface. The optic cup forms a deep pit, in the mouth of which is placed the lens. The retinal layers are well developed (cf. Langerhans), and the outer layer of the optic cup or layer of retinal pigment (_rp_) contains numerous pigment granules, especially on its dorsal side. At the edge of the optic cup the two layers fall into each other. They constitute the commencement of the pigment layer of the iris; but at this stage they are not pigmented. The mesoblast of the iris is hardly differentiated. The lens (_l_) has the normal structure of the embryonic lens of Vertebrata. The inner wall is thick and doubly convex, while the outer wall, which will form the anterior epithelium, is very thin. There is a large space between the lens and the retina containing the vitreous humour (_v.h_). There is no aqueous humour, and the tissues in front of the lens bear but little resemblance to those in higher Vertebrata. The cornea is represented by (1) the epidermis (_ep_); (2) the dermis (_d.c_); (3) the subdermal connective tissue (_s.d.c_) which passes without any sharp line of demarcation into the dermis; (4) a thick membrane continuous with the choroid which represents Descemet's membrane. The subdermal connective tissue is continued as an investment round the whole eye. There is no specially differentiated sclerotic, and a choroid is only imperfectly indicated[31]. The peculiar features of the eye of the young larva of the Ammocoetes are probably due to degeneration.