STRUCTURE OF FISHES. 413 



rating the trunk into a thoracic and caudal portion. To 

 make up for the short intestine, its absorbing surface is 

 greatly increased in all except the bony fishes by a peculiar 

 fold called the "spiral valve." The rectum always opens in 

 front of the urinary and genital outlets ; except when the 

 latter communicates directly with the rectum, thus forming 

 a cloaca. All fishes have a well-developed liver, usually a gall- 

 bladder, with several gall-ducts ; and in general a yellowish 

 pancreas. 



The heart consists of a ventricle and auricle, the latter 

 branchial with a venous sinus (sinus venosus) ; while to the 

 ventricle is added an arterial bulb, Avhich subdivides into five 

 pairs of arteries, one for each gill-arch. The Dipnoi ap- 

 proach the Amphibians in the possession of a second auricle 

 as well as of genuine lungs, i.e., cellular air-sacs. The lungs 

 are fundamentally comparable with the air-bladder or .swim- 

 ming bladder. It is generally situated below the back-bone, 

 and is developed originally as an offshoot of the oesophagus. 

 It is either free, not connected with the digestive tract, or its 

 original attachment may be retained in the form of the 

 "pneumatic duct," which, when persistent, opens into the 

 oesophagus. In the sharks it is either absent or exists in a 

 rudimentary state. 



The kidneys are two voluminous, dark-red lobulated or- 

 gans, lying close together next to the back-bone, behind, i. c., 

 above the air-bladder, and occupying nearly the whole length 

 of the abdominal cavity. The efferent ducts (ureters) either 

 pass along in front of or by the side of the kidney, and some- 

 times unite to form a single sac, the outlet of which is situ- 

 ated either behind or below the generative orifice. It has 

 been found that the minute structure of the kidneys of em- 

 bryo sharks resembles somewhat the segmental organs of 

 worms, the original kidney being composed of bundles of 

 ciliated funnels, like those of worms, combined, however, 

 with Malpighian bodies and renal lobules which do not exist 

 in worms, while all these parts have a common duct, the 

 ureter, which also does not exist in worms, being character- 

 istic of Vertebrates. 



In the fishes the sexes are, with a very few exceptions, dis- 



