48 VERTEBRATE ZOOLOGY 



A detailed study of the organogeny of Amphioxus would not fall 

 within the scope of the present volume, but brief mention should be 

 made of a few of the later stages, and of the more significant changes 

 involved. A stage of much interest is shown in Fig. 20. This is an 

 embryo showing nine pairs of primitive somites and in process of 

 budding off the right and left head cavities. Up to this point the 

 embryo is strictly bilaterally symmetrical. The first disturbance of 

 bilaterality is seen in connection with the head-cavities, for the right 

 one grows large and becomes the cavity of tlie snout or pre-oral body 

 cavity, lying beneath the notochord, and the left becomes a rudimen- 

 tarj' structure, the pre-oral pit. In Tig. 20, C is shown the beginning of 

 the overgrowth of the right head cavity. This crowding over of the 

 right side of the head toward the left disturbs the primitive location 

 of the mouth, so that it opens to the left side and only secondarily 

 adjusts itself to the nearly median postition characteristic of the 

 adult. The gill-slits, myotomes, spinal nerves, nephridia, and other 

 metameric structures are thrown out of the primitive paired arrange- 

 ment and alternate on the two sides throughout life. In the genus 

 Asymmetron the asymmetry of the whole body is more pronounced 

 than in the more typical members of the Amphioxus family; hence 

 the name. This twisting about of the body is a characteristic of 

 sessile animals and it is significant to find in Amphioxus a condition 

 comparable with that seen in the echinoderms, which are entirely 

 headless creatures, and in Balanoglossus in which the head is greatly 

 reduced. 



More advanced larvae of Amphioxus show the migration of the 

 mouth from the left side to the middle, the method of uniting the 

 primary gill-slits into the definitive double type separated by tongue 

 bars, the formation of the oral hood and buccal cirri. There is no sud- 

 den change equivalent to metamorphosis, but at this time the larva 

 gives up its free-swimming habit and begins to lead the semi-seden- 

 tary, sand-burrowing life of the adult. The period of adolescence is 

 long and slow and culminates in the maturing of the gonads. 



SUB-PHYLUM IL UROCHORDATA (TUNICATES) 

 Just as there is no doubt about the affinities of Amphioxus to the 



vertebrates, so there is no question but that the tunicates are related 



to Amphioxus. 



In contrast with the cephalochordates, in which one family of two 



