80 



METAMORPHOSIS. 



surface of the head. The lumen of the sack is well developed; and lies 

 in contact with the base of the fore part of the brain. 



The vascular system presents no very remarkable features. The heart 

 is two-chambered and straight. The ventricle is continued forwards as 

 a bulbus arteriosus, which divides into two arteries at tlie thyroid body. 

 From the bulbus and its continuations eight branches are given off to the 

 gills; and as mentioned above a vessel, probably of the same nature, 

 is given off in the region of the velum. The blood from the bianchial 

 sacks is collected into the dorsal aorta. Some of it is transmitted to the 

 head, but the greater part flows backwards under the notochord. 



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 

 sub-intestinal 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. Miiller (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 develop- 

 ment. 



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 sep- 

 tum. The eye (fig. 48) is much 

 more fully develojjed, 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 de- 

 veloped (cf. Langerhans) and the 

 outer layer of the optic cup or 

 layer of retinal pigment (rp) con- 

 tains numerous pigment granules 

 especially on its dorsal side. At 

 the edge of the optic cup the two 

 layers fall into each other. They 



Fig. 48. Eye of an Ammocoetes lying 

 beneath the skin. 



ep. epidermis ; d.c. dermal connective 

 tissue continuous with the sub-dermal con- 

 nective tissue (s.d.c), which is also shaded. 

 There is no definite boundary to this tissue 

 where it surrounds the eye. 



m. muscles ; dm. membrane of Descemet ; 

 I. lens ; v.h. vitreous humor ; r. retina ; rp. 

 retinal pigment. 



