BODY CAVITY. 



415 



The pharynx opens into the intestinal region of the gut 

 which is straight and simple. Near its commencement a 



pouch-like "liver" or caecum 

 (Fig. 133, C.) arises, and ex- 

 tends forward on the right side 

 of the pharynx. The anus is 

 some distance from the end of 

 the body (cf. Fishes) ; in the 

 larva it is close to the caudal 

 fin. 



Body Cavity. This can only be 

 understood when its development 

 is studied. From the archenteron 

 of the embryo a pouch grows out 

 on each side, and becomes almost 

 at once segmented into a series of 

 small sacs. These lie one behind 

 the other, and soon lose all con- 

 nection with the gut. Each ulti- 

 mately divides into two portions, 

 an upper, the true primitive seg- 

 ment, and a lower, corresponding 

 to the lateral plate of other Verte- 

 brates. The primitive segments 

 form the body musculature, and 

 retain their segmentation. Their 

 cavity, the myocoele, persists to 

 some extent in the adult, forming 

 the system of lymph spaces and 

 canals which lie below the cutis. 

 In the region of the lateral plates 

 the septa disappear, and the en- 

 closed spaces, bounded by somato- 

 pleure and splanchnopleure, unite 

 to form the "splanchnocoele" which 

 surrounds the gut. Posteriorly, this 

 space exists unconstricted in the 

 adult ; anteriorly, it is reduced to 

 small spaces and ccelomic canals by 

 the development of the atrial cham- 

 ber. This pushes the somatopleure 

 up before it as it develops, and so 

 is hollowed out at the expense of the true ccelome. The ccelomic 

 spaces and canals contain coagulable fluid, and are in some regions 

 continuous with the blood vessels. They represent the lymphatic 

 system of higher forms. 



FIG. 134. Cross section of 

 Amphioxus through the gill 

 slit region. (After BOVERI and 

 HATSCHEK.) 



The spinal cord, nolochord, pharynx, 

 and atrial cavity are unlettered. 



, Sclera layer ; />, fascia ; c, muscle 

 plate ; d, cutis ; e, nephridial canal ; 

 f, traverses the sub-chordal part of the 

 coelome on left side ; g; glomerulus of 

 kidney tube ; 7z, gonad ; ^metapleural 

 cavity ; j\ transverse muscle ; k, cavi- 

 ties in junction of metapleural folds ; 

 /, cavity in dorsal fin ; m, aorta ; n, 

 branchial vessel ; <?, branchial artery, 

 the line traverses first the metapleural 

 cavity, then the atrial cavity, and finally 

 a minute part of the coelome beneath 

 the pharynx. 



