XVII 



PROTOCHORDATA 



595 



into right and left cavities, of which the right is larger than the left. 

 These are the so-called head-cavities, which we have compared to the 

 proboscis-cavity of Enteropneusta. The remainder of the archenterou 

 now constitutes the gut. 



Meanwhile the somites derived from the trunk-cavities become 

 divided into upper portions, termed myotomes, from whose inner 

 walls the muscles are formed, and lower portions, which fuse with 

 their predecessors and successors to form the general body-cavity or 

 splanchnocoele. The right and left splanchnocoeles meet with one 

 another and fuse into one cavity in the mid-ventral line beneath 

 the gut. Above the gut they remain separate from one another, and 



arch 



FIG. 433. Illustrating the formation and development of the head-cavities 

 Amphioxus lanceolatus. 



A, transverse section through the anterior region of a larva with six myotomes the head-cavities are 

 seen to be developing as outgrowths of the front end of the gut (archenteron). B, transverse section 

 through the anterior region of a larva two days old just before the mouth opens the left head-cavity is 

 a thick-walled vesicle ; the right head-cavity is a thin-walled vesicle and lies beneath the left one. arch, 

 archenteron (gut) ; ch, notochord ; ex, vacuolation of ectoderm where the external opening of the left 

 head-cavity will be formed later ; Lcoll, left collar-cavity ; l.h, left head-cavity ; n.t, nerve tube ; r.coll, 

 right collar-cavity ; r.h, right head-cavity. 



between them lies a blastocoele space, double in front but single 

 behind, which is the rudiment of the dorsal aorta. 



Since new somites are being continually cut off from the hinder 

 parts of the trunk-cavities, all stages in the development of a somite 

 may be made out in one and the same individual. The somites derived 

 from the collar-cavities, however, do not become subdivided like the 

 rest, nor do they fuse with the splanchnocoeles. In early larval life, at 

 any rate, these lower parts constitute cavities lying at the ventro- 

 lateral regions of the animal, outside the splanchnocoeles, whilst the 

 upper portions of the collar -cavities, above the myotome region, 

 extend forwards at the sides of the nerve cord. The general form of 



