CEPHALOCHOEDA 



149 



projection is accompanied by a prolongation of the notochord 

 beyond its original anterior limits, and by hollow outgrowths 

 of the first pair of myocoels. At this period also the anterior 

 end of the enteron is constricted off and divides below the 

 proboscis into two cavities. These are the right and left head 

 cavities. These coelomic spaces, developed independently of 

 the rest of the coelom, are with good reason believed to repre- 

 sent in Amphioxus the head cavity or cavities of the Hydro- 

 coela. Their fate in Amphioxus is of interest. The right 

 cavity increases, and its epithelium becomes flattened to form 

 a coelomic space in front of the enteron and below the noto- 

 chord. It remains as a closed cavity. The left cavity retains 



Left 

 Right head cavity pore cavity Mouth Club gland Splanchnocoel 



1st gill 



FIG. 73. Amphioxus. Head of larva, to show the relationship of the first 

 three myocoels to the splanchnocoel and position of the head cavities. 



its cylindrical ciliated epithelium and remains a small 

 cavity. At the beginning of the asymmetrical larval period 

 it establishes an opening to the exterior on the left side in 

 front of the mouth, and is thus converted into the preoral pit. 

 It is gradually carried to the median position below the noto- 

 chord, and the ectodermal part of it increased by a groove. 

 It becomes associated with ciliated folds of the oral cavity 

 which form the wheel organ. 



It will be plain from the foregoing account and the figures 

 that the development during the early symmetrical phase is 

 very different from that of the Invertebrates. Up to the 

 time of the formation of the gastrula there is nothing to remark 

 except the absence of mesenchyme. Thereafter the develop- 

 mental changes are different in every respect. The dorsal 



