XX] CIRCULATORY SYSTEM 487 



The olfactory lobes are sometimes connected with the cerebrum 

 by very long stalks (Cod family), but at other times they are sessile 

 (most other Teleostei). 



In the blood system the main difference between Teleostei and 

 Chondrichthyes lies in the fact that in the former group the con us 

 of the heart has disappeared as a distinct chamber, since it becomes 

 merged in the ventricle, which has thereby become enlarged. There 

 is only one transverse row of pocket valves, and these are situated 

 at the origin of the ventral aorta, and correspond to the most distal 

 pair in Chondrichthyes. The ventral aorta is thickened at its origin 

 by an increase of fibrous tissue, but this swelling, termed the 

 bulb us arteriosus, is not rhythmically contractile. Besides this 

 peculiarity of the heart we find that in Teleostei the four epibran- 

 chial arteries on each side, instead of converging to join the dorsal 

 aorta, join a ring-shaped vessel termed the circulus cephalicus, 

 from which two posterior carotid arteries are given off in front 

 to supply the brain and from which behind the dorsal aorta arises. 

 The efferent branchial vessels do not form loops round the gill-clefts 

 as in Chondrichthyes, but the efferent vessel from the hinder wall of 

 the first cleft, i.e. the cleft between the hyoid and first branchial 

 arches, gives off a branch which runs round the ventral end of the 

 cleft and carries blood to the pseudobranch. From the pseudo- 

 branch blood is carried by an ophthalmic artery, which runs 

 forward to supply the vascular choroid investment of the eye, and 

 is connected with its fellow by a bridge above the parasphenoid. 

 From this bridge go off arteries to the brain in front of the posterior 

 carotids. These branches of the ophthalmic arteries are known as 

 anterior carotids (Fig. 239). 



The egg of the Teleostean fish develops into a larva which, 

 although showing special characters in each family of the group, has 

 certain general characters as well. Thus it possesses a continuous 

 fin-fold in place of the separate dorsal caudal and anal fins of the 

 adult. The notochord is at first quite unconstricted, because there 

 are no centra, and it runs straight to its termination : the end is 

 not bent up. Pectoral fins alone are developed, and the excretory 

 organ is a pronephros, consisting of a single closed chamber on 

 each side, from the inner wall of which is developed a glomerulus. 

 From this chamber a single pronephric tubule leads into the archi- 

 nephric duct. When the adult form has been attained the fish is 

 far from having reached the adult size, and in general several years 

 must elapse before sexual maturity is attained. 



