152 ESSENTIALS OF ZOOLOGY 



each side of the body, the mouth is rotated forwards, and two 

 oral folds develop which form the oral hood, and the original 

 mouth is narrowed to become the velum. The gills are pro- 

 tected by folds of the ectodermal wall which are produced 

 on each side of a ventral groove and fuse beneath them to 

 enclose the atrial cavity. Just before their formation ecto- 

 dermal invaginations at the morphological upper part of the 

 gills grow inwards to be developed into the protonephridia. 



The animal is now ready to begin its burrowing life, and 

 as it grows the gills are increased in number posteriorly. 

 During the growth the last trace of the neurenteric canal 

 disappears and a short tail is formed into which the endoderm 

 does not penetrate, but all the other elements of the dorsal 

 lip are produced. 



It has already been remarked that one of the peculiarities 

 is the absence of mesenchyme. Such is formed during growth 

 from the coelomic mesoderm and gives rise to the connective 

 tissues other than those which are primarily developed from 

 the myocoel. The primary body cavity is represented mainly 

 in Amphioxus by the very primitive blood and blood circu- 

 latory system, and it is to this that the solenocytes of the 

 protonephridia are related. The predecessors of Amphioxus, 

 the Hydrocoela, possessed a water cushion as an anterior buffer. 

 The water cushion is certainly developed in Amphioxus in the 

 head cavity, but it is reinforced by the gelatinous skeleton 

 of the notochord which is carried forwards to support the 

 proboscis. Another feature of interest is the concentration 

 of the veins to the meeting-place behind the gills, and the hint 

 of a heart there in the region where the blood is propelled 

 into the gills. 



