CHAPTER XI.. Urinary Organs.

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.

The kidneys of Teleosteans are situated likewise without the peritoneal cavity, immediately below some part of the vertebral column, and vary exceedingly with regard to form and extent. Sometimes they reach from the skull to between the muscles of the tail, sometimes they are limited to the foremost part of the abdominal cavity (in advance of the diaphragm), but generally their extent corresponds to that of the abdominal portion of the vertebral column. Frequently they are irregular on their dorsal surface, filling every available recess, flat, attenuated on the sides, more or less coalescent towards the middle; in other fishes they are more compact bodies. The ureters terminate, either separate or united, in a urinary bladder, varying in shape, which opens by a short urethra behind the vent. The urinary opening may be separate or confluent with that of the genital ducts, and is frequently placed on a more or less prominent papilla (papilla urogenitalis). If separate, the urinary opening is behind the genital; and if a papilla is developed, its extremity is perforated by the urethra, the genital opening being situated nearer the base. A few Teleosteans show an arrangement similar to that of Chondropterygians and Dipnoi, the urogenital openings being in the posterior wall of the rectum (Symbranchidæ, Pediculati, and some Plectognathi).