64 



into a slender filament, wliile the nucleiTS alters in properties, imljibing stains sucli as 

 carmine more intenselj^ and forms what is called the head of the spermatozoon. 

 During the spawning period all the testicular cells, except the large cells of the 

 male germinal epithelium at the closed ends of the testicular tubes, are converted into 

 spermatozoa, which are couvej'ed to the exterior suspended in a liquid similar to. 

 lymph. This liquid is produced within the testis, being probably exuded from 

 the blood vessels and lymph of the testicular stroma into the testicular tubes. The 

 liquid containing the spermatozoa is usually called the milt. When the spermatozoa 

 reach the sea water they swim about actively, and when one meets a ripe ovum it 

 enters its substance and fertilises it. 



The Vascular System. 



The blood vessels ramify through all the tissues, including the skin and nervous 

 system, which have not yet been particularly described. But as the heart, which is the 

 central organ of the vascular system, is closely related to the body cavity and to the 

 branchiae, the system may be conveniently described here. 



The heart, as was before mentioned, is enclosed in a special chamber of its own, 

 which is reaUy a part of the body cavity, though it is entirely shut off from the visceral 

 cavity in which the organs of digestion, &c., are contained. This cavity containing the 

 heart, the pericardium, is situated between the posterior and internal walls of the two 

 branchial chambers ; the cavity is wedge-shaped, the edge of the wedge being anterior, 

 the base posterior. Posteriorly the pericardium is separated from the visceral 

 cavity by a thin membrane which lies in front of the liver and below the gullet or 

 oesphagus. Outside the pericardium on either side is the bone called the clavicle. 

 The heart itself is conical in shape, the point being ventral and directed downwards 

 and slightly forwards, the broad base being dorsal. It is hoUow and divided into two 

 portions connected by a valve. The dorsal portion has very thin walls, and is called 

 the auricle ; the ventral portion has thick muscular walls, and is called the ventricle. 

 Into the dorsal end of the auricle open three large veins by which the blood which has 

 passed through the various parts of the body is conducted into the cavity of the 

 auricle. These veins are the ductus Cuvieri which pass one on either side of the 

 oesophagus from the kidnej's, and the hepatic vein which passes forwards from the liver 

 below the oesophagus. The ductus Cuvieri of each side is formed by the union of two 

 veins, one running forwards in the substance of the kidney and conveying the blood 

 from the trunk, the other running backwards from the head and conveying the blood 

 from the brain, skull, &c. ; these are called the anterior and posterior cardinal 

 veins. 



The ventricle sends off a single main blood vessel, the ventral aorta. At the opening 

 of this vessel from the ventricle, there are three internal folds or valves which prevent 



