In the majority of Teleosteans the _aorta_ has proper walls formed by its own membranes, but in the Sturgeons it is independent at its commencement only, and replaced by a canal formed by hæmal elements of the vertebral column, and clothed inside with a perichondrium. In many Chondropterygians and some Teleosteans (_Esox_, _Clupea_, _Silurus_), the aorta possesses its own firm membranes along its ventral side, dorsally being protected by a very thin membrane only, attached to the concavity of the centra of the vertebræ. The circulatory system of _Branchiostoma_ and of the _Dipnoi_ shows essential differences from that of other fishes. _Branchiostoma_ is the only fish which does not possess a muscular heart, several cardinal portions of its vascular system being contractile. A great vein extends forwards along the caudal region below the notochord, and exhibits contractility in a forward direction; it is bent anteriorly, passing into another tube-like pulsatile trunk, the branchial heart, which runs along the middle of the base of the pharynx, sending off branches on each side to the branchiæ; each of these branches has a small contractile dilatation (_bulbillus_) at its base. The two anterior branches pass directly into the aorta, the others are branchial arteries, the blood of which returns by branchial veins emptying into the aorta. The blood of the intestinal veins is collected in a contractile tube, the portal vein, situated below the intestine, and distributed over the rudimentary liver. Of all other fishes, only in _Myxinoids_ the portal vein is contractile. All the blood-corpuscles of _Branchiostoma_ are colourless and without nucleus. In _Dipnoi_ a rudimentary division of the heart into a right and left partition has been observed; this is limited to the ventricle in _Ceratodus_, but in _Lepidosiren_ and _Protopterus_ an incomplete septum has been observed in the atrium also. All Dipnoi have a pulmonal vein, which enters the atrium by a separate opening, provided with a valve. The pulmonal artery rises in _Lepidosiren_ and _Protopterus_ from an arch of the aorta, but in _Ceratodus_ it is merely a subordinate branch, rising from the _Arteria cœliaca_. CHAPTER XI. URINARY ORGANS. In _Branchiostoma_ no urinary organs have been found. In Myxinoids these organs are of a very primitive structure: they consist of a pair of ducts, extending from the urogenital porus through the abdominal cavity. Each duct sends off at regular intervals from its outer side a short wide branch (the uriniferous tube), which communicates by a narrow opening with a blind sac. At the bottom of this sac there is a small vaso-ganglion (_Malpighian corpuscle_), by which the urine is secreted. In the Lampreys the kidneys form a continuous gland-like body, with irregular detached small portions. The ureters coalesce before they terminate in the urogenital papilla.