IV. VERTEBRATA. 48i 



coelom is connected with the exterior by one or two openings (pori ab- 

 dominales) beside or behind the anus. 



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) cavity. The lining 

 membranes of these cavities are called pleura and peritoneum respectively. 

 The pericardial cavity is also a derivative of the coelom, and the lining, 

 the pericardium, but a part of the pleuroperitoneal membrane. Hence it is 

 that in many fishes (sharks, sturgeon) a communication persists between 

 the pericardial and the other coelom. 



Of the vegetative organs, the alimentary tract possesses the greatest 

 systematic interest for it not only is concerned with digestion, but fur- 

 nishes, as in all chordates, the respiratory 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, which are lined 

 with the soft 'mucous membrane' much the same in character as the 

 entodermal part of the canal. 



The first division is spacious and consists of the ectodermal mouth 

 cavity and the entodermal pharynx, two spaces which, in most vertebrates, 

 are not sharply marked off, but in alligators and mammals are separated 

 by the soft palate. Then, begins the narrow oesophagus, widening behind 

 to the stomach. From the hinder or pyloric 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 oeca. 

 The terminal portion in most vertebrates is called the cloaca because it 

 receives the urogenital ducts. The liver is the only gland constantly 

 present; it is a large compact organ, generally provided with a gall bladder. 

 Usually a smaller gland, the pancreas, occurs. The ducts of the liver 

 (bile duct, due/us choledoc/nis) 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 has blind sacs and glands. 



A striking vertebrate characteristic occurs in the dentition. In the 

 cyclostomes there are horny teeth strongly cornified epithelial products 

 totally distinct from the true teeth of dentine and enamel of the higher 

 groups, which occur in places where the underlying skeleton affords them 

 a firm support, especially on the upper or lower jaws, but they may occur 

 on other bones of the mouth and pharyngeal cavities (roof of the mouth, 

 gill arches). They have apparently arisen from a diffuse dentition, re- 

 calling the scales of the skin, since many elasmobranchs possess, besides 

 31 



