ALIMENTARY SYSTEM—BODY CAVITY. 503 
ALIMENTARY SySTEM.—SUMMARY 
REGION OF THE GuT. OuTGROWTHs. ASSOCIATED STRUCTURES. 
Mouth cavity, Oral part of the} Teeth. 
or Stomodzum, hypophysis. Salivary glands. i 
or Fore-gut, Tongue. : 
originating as an ectodermic 
invagination. 
Pharynx, gullet or ceso-| Thyroid\and the| With the several out- 
phagus, stomach, small in-}| Thymus f gill-clefts. | growths the surrounding me- 
testine, large intestine, and| Air bladder; lungs. | soderm becomes associated, 
rectum ;=the mesenteron or| Liver. often to a great extent. 
mid-gut, originating from} Pancreas. Note also the origin of 
the cavity of the gastrula, Allantois. the notochord as an axial 
the archenteron or primitive] The pancreas is | differentiation of cells along 
gut; lined by endoderm. usually the result of | the mid-dorsal line of the 
two ventral © out- | embryonic gut. 
growths and a dorsal 
one. In Cyclostomes 
Anal region, and Elasmobranchsit | In some Fishes, all Amphi- 
or Proctodzum, seems to have but bians, all Sauropsida, and 
or Hind-gut, one rudiment; in the | the Prototherian Mammals, 
originating as an ectodermic | Sturgeon four. the terminal part of the 
invagination. gut is a cloaca or common 
chamber, into which the 
rectum, the urinary, and the 
genital ducts open. 
Body cavity.—In Amphioxus the ccelom arises as pouches 
from the archenteron (enzerocelic). In the other Vertebrates, 
owing to modified processes of development, probably first 
arising from the presence of much yolk, solid cell masses 
grow out in place of hollow sacs, but the cavities which 
appear later, apparently by splitting of the cell mass 
(schizocelic), are in reality the retarded cavities of true 
coelom-pouches. A dorsal segmented portion (protoverte- 
bree) becomes separated off from a ventral unsegmented 
portion (Fig. 255). It is this ventral portion which forms 
the body cavity of the adult. In the adult it is divided 
into an anterior pericardial and a posterior peritoneal 
portion. 
The body cavity may form part of one or all of the following systems : 
—(1) excretory, voiding waste by abdominal pores or by nephrostomes ; 
(2) reproductive, receiving the liberated genital elements; and (3) 
lymphatic, receiving transudations from visceral and abdominal organs. 
