26 ANATOMY OF AMPHIOXUS. 



the caudal fin, and the right margin of the oral hood is 

 shown to be continued round the front end of the body 

 into the cephalic expansion of the dorsal fin. 



Ccclom. 



The question now arises : if the atrial cavity is no: 

 the true body-cavity, what has become of the latter ? In 

 order to determine this point, it is necessary to have 

 recourse to transverse sections through the body, such as 

 the one represented in Fig. 2, which is taken through the 

 middle of the pharyngeal region. In a section like this, 

 the work of tracing the limits of the atrial cavity is often 

 greatly facilitated by the presence of a rich brown pigment 

 in the epithelium lining its walls. We find, accordingly, 

 that the atrial cavity has extended itself at the expense of 

 the coelom, and has reduced the latter, in the main, to a 

 small space on either side of the dorsal aorta, the aorta 

 being double in this region (Fig. 2). This portion of 

 the coelom is sometimes spoken of as the snpra-pharyngeal 

 ccelom, and sometimes as the snbcJwrdal coelom, since it lies 

 dorsal to the pharynx on the one hand, and below the noto- 

 chord on the other. Other fragments, so to speak, of the 

 coelom are found accompanying some of the branchial bars, 

 namely, every alternate one ; and another portion occurs 

 below the endostyle. (See Fig. 13.) The hepatic coecum 

 is also surrounded by a division of the coelom, but its 

 cavity is reduced to a minimum, and the same applies to 

 the ccelom surrounding the intestine immediately behind 

 the pharynx. Behind the atriopore, as we have seen, the 

 atrial cavity is confined to the right side, so that on the 

 left side of the intestine in this region the coelom presents 

 its original proportions. 



