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.
In Chondropterygians the kidneys occupy the posterior half or two-thirds of the back of the abdominal cavity, without the sac of the peritoneum (as in all fishes) which forms a firm tendinous horizontal septum. The kidneys of the two sides are never confluent, and generally show a convoluted or lobulated surface. The ureters are short; each is dilated into a pouch, and communicating with its fellow terminates by a single urethra (which also receives the vasa deferentia) behind the end of the rectum in the large common cloaca.
In Ganoids the kidneys occupy a similar position as in Chondropterygians, but these fishes differ considerably with regard to the termination and the arrangement of the ends of the urogenital ducts. The Dipnoi possess a cloaca. In Ceratodus the ureters open into it by a common opening, separate from the genital opening; and no closed urinary bladder has been developed. Lepidosiren has a small urinary bladder; the ureters do not communicate directly with it, but terminate separately on small papillæ in the dorsal compartment of the cloaca. The other Ganoids lack a cloaca, and the urogenital opening is behind the vent as in Teleosteans. In all the genital and urinary ducts coalesce towards their end. The Sturgeons have no urinary bladder, whilst it is present in Amia, the ureters opening separately into it.