THE DEVELOPMENT OF AMPHIOXUS 55 



completely divided. The liver or caecum is just beginning to 

 develop. 



At this time the larva largely gives up its free swimming 

 habit, and assumes the adult habit of burrowing in the sand 

 or mud of the bottom, frequently remaining buried with 

 only the anterior end protruding. The cilia of this exposed 

 region, and of the pharynx in general, vibrate actively, carry- 

 ing into the pharynx a respiratory current containing small 

 nutritive organisms. The age of the larvae at the critical stage 

 varies greatly but three months may be taken as a rough 

 approximation. 



IV. THE ADOLESCENT PERIOD 



The larva now enters upon its long period of adolescence. 

 This is characterized by the very gradual assumption of adult 

 characteristics, chiefly through histological differentiations and 

 increasing complexity of many regions of the body. The devel- 

 opment of the brain and cord has been mentioned. In the 

 pharynx pairs of gill slits (tertiary) are slowly added and the 

 pharyngeal wall assumes the complex structure of the adult. 

 Apparently gill slits are added slowly throughout life, and 

 usually number upward of one hundred pairs in mature speci- 

 mens. The liver pushes forward as a simple blind sac, into the 

 atrial cavity on the right side, carrying before it a fold of fused 

 atrial and coelomic walls; finally it extends far forward into the 

 pharyngeal region. The most important development of the 

 period is the formation of the gonads. 



Gonads 



In reality the gonads begin to develop before the end of the 

 larval period. They appear in the ninth or tenth segment and 

 continue to about the thirty-fifth. They are first indicated 

 as small groups of cells in the floor of the myoccel, in the re- 

 gion where the skeletogenous layer passes into the cutis layer. 

 This is the region of the nephrostome in the Craniates, and the 



