THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 159 



described in connection with the section on chordate development, Section IV. This should 

 be re-read and Figures 7, 8, and 10 studied. (See also Fig. 44(7; K, Figs. 8 and 9, pp. 14 and 

 15; W, Fig. 15, p. 65.) 



2. Divisions of the coelom. At first the coelom consists of a continuous cavity extending 

 the entire length of the trunk region, divided into two longitudinal halves by the dorsal and 

 ventral mesenteries. With the partial disappearance of the ventral mesentery, the two 

 halves of the coelom are connected ventral to the intestine (Fig. 44(7). In the adults of all 

 vertebrates the coelom is divided into at least two compartments by the formation of a par- 

 tition. This partition, called the transverse septum, develops at the posterior end of the heart 

 and cuts off the heart from all of the other viscera. The transverse septum thus divides the 

 coelom into a small anterior compartment, the pericardial cavity, which contains only the 

 heart, and a very large posterior compartment, the pleuroperitoneal cavity, which contains all 

 of the other viscera (see Fig. 45^). The pericardial cavity in fishes and urodeles is anterior 

 to the pleuroperitoneal cavity, and the transverse septum in those groups passes transversely 

 across the body (Fig. 45^ and B). In the Anura and all vertebrates above Anura the peri- 

 cardial cavity has descended posteriorly so that it comes to lie ventral to the anterior part of 

 the pleuroperitoneal cavity; the transverse septum then assumes an oblique position (Fig. 456"). 

 In that portion of the pleuroperitoneal cavity which in consequence of the descent of the 

 pericardial cavity lies dorsal to the pericardial cavity, the lungs are situated. This condition 

 of the coelom, as in Figure 456", is found in Anura and reptiles. In birds and mammals, the 

 pleuroperitoneal cavity is divided into anterior and posterior parts by the formation of a parti- 

 tion which descends from the dorsal body wall and unites with the transverse septum (Fig. 45/2 

 and E). This partition is known as the oblique septum in birds and as the diaphragm in mammals. 

 In birds it is non-muscular while in mammals it is infiltrated with striated muscles derived from 

 adjacent myotomes of the body wall. 



The oblique septum or diaphragm forms immediately posterior to the lungs. That 

 portion of the pleuroperitoneal cavity which is cut off anterior to the oblique septum or 

 diaphragm consequently contains the lungs. It consists of the two pleural cavities or pleural 

 sacs, each inclosing a lung. The two pleural cavities are completely separated from each 

 other, the pericardial cavity containing the heart being situated in the median line between 

 their ventral portions. That part of the pleuroperitoneal cavity cut off posterior to the 

 oblique septum or diaphragm is called the peritoneal or abdominal cavity; it incloses the 

 greater part of the digestive tract and the urogenital system. It will be seen from this 

 account that in birds and mammals the coelom is divided into four compartments the 

 pericardial cavity, the two pleural cavities, and the peritoneal cavity. 



It is convenient to speak of the viscera as being inclosed in or contained in the coelomic 

 cavities. This is not, however, a correct expression, since, owing to the fact that the viscera 

 are covered by the visceral peritoneum or serosa, they are not really inside of the coelom in the 

 same sense that a chair could be said to be inside of a room. They are outside of it and have 

 the same relations to it as if they were pushed into the coelom carrying the coelomic wall before 

 them. To illustrate farther, one cannot get into the inside of a tent or a balloon by pushing 

 against the wall; one carries the tent or balloon wall before him and always remains in reality 

 on the outside of the tent or ba!loon. Similarly, the viscera are outside of the coelom although 

 they appear to be contained within its cavity. 



B. THE DIGESTIVE TRACT AND ITS DERIVATIVES 



i. The origin of the digestive tract. The primitive intestine or archenteron, as we 

 learned in the section on development, is produced by the process of invagination or other 

 processes in the gastrula stage of the embryo. It is at first a simple tube of entoderm with one 



