CYCLOSTOMATA. 97 



supply nutriment for the growth of the embryo, and although in the anterior 

 part of the intestine they become to some extent enclosed in the alimentary 

 tract and break up, yet in the posterior part they become wholly transformed 

 into the regular epithelium of the intestine. 



On the ninth day a slight fold filled with mesoblastic tissue is visible on 

 the dorsal wall of the intestine. This fold appears to travel towards the 

 ventral side ; at any rate a similar but better-marked fold is visible in a 

 ventro-lateral position at a slightly later period. This fold is the com- 

 mencement of the fold which in the adult makes a half spiral, and is no doubt 

 equivalent to the spiral valve of Elasmobranchs and Ganoids. It contains 

 a prolongation of the coeliac artery, which constitutes at first the vitelline 

 artery. 



The nervous system does not undergo during the early larval period 

 changes which require a description. 



The opening of the olfactory sack becomes narrowed and ciliated (fig. 47, 

 <?/). It is carried by the process already mentioned to the dorsal 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 the 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 branchial 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 development. 



The branchial skeleton becomes more developed, and capsules for the 

 olfactory sack and auditory sacks are established. 



B. III. 7 



