546 CIIORBATA. 



whole length from the vertel)ral cohimn, but ventral of the diges- 

 tive tract (as the mediastinnm, omeiitum minus, and susjjensory 

 ligament of the liver of human anatomy) only reaches as far back 

 as the liver, so that right and left cceloms unite behind. Some 

 other organs are also suspended in the body cavity by membranes: 

 the testes by the mesorchium, the ovary by the mesovarium. 



The body cavity is frequently called the pleuroperitoneal cavity, 

 since in mammals it is divided by a partition, the diaphragm, into 

 an anterior or pleural and a posterior or peritoneal (abdominal) 

 cavitjr. The lining membranes of these cavities are called pleura 

 and j^eritoneuni respectively. The j^ericardial cavity is also a de- 

 rivative of the cwlom, and the lining, the jjericardium, but a part 

 of the pleuro])eritoneal membrane. Hence it is that in many 

 fishes (sharks, sturgeon) a communication persists between the 

 2)ericardial and the other coslom. In most fishes and in many rep- 

 tiles there is a direct connexion of the coelom with the exterior by 

 one or two 2:iori abdominales, beside or behind the anus. 



The alimentary tract possesses the greatest S3fstematic interest 

 of tlie vegetative organs, for it not only is concerned with diges- 

 tion, but furnishes, as in all chordates, the resjiiratory organs (gills 

 and lungs) as well, these arising in the non-chordates from the 

 ectoderm. It begins with the anterior ventral mouth and ends 

 ventrally with the anus, some distance in front of the tip of the 

 tail; it is almost wholly entodermal in origin, there being but 

 slight ectodermal portions at either end. 



The first division is spacious and consists of the ectodermal 

 mouth cavity and the entodermal 25harynx, two spaces which, in 

 most vertebrates, are not sharply marked oif, but in alligators and 

 mammals are separated by the soft palate. Tlien begins the 

 narrow oesojjhagus, widening behind to the stomach. From the 

 hinder or i)yloric end of the stomach begins the small intestine, 

 which enlarges into the large intestine, separated from the small 

 intestine in the higher vertebrates by a valve and one or two cwca. 

 The terminal portion in most vertebrates is called the cloaca be- 

 <;ause it receives the urogenital ducts. The liver is the only gland 

 constantly present; it is a large compact lu'own organ, generally 

 provided with a gall Jiladder. ITsually a smaller gland, the 

 pancreas, occurs. The ducts of the liver (bile duct, ductus 

 choledochus) and pancreas empty into the small intestine near the 

 pylorus. The mouth cavity may have salivary glands connected 

 with it, while the rectal region occasionally lias blind sacs and 

 glands. 



