XIII 



PHYLUM CHORDATA 



35 



sense-vesicle (sens. res.). The posterior Barrow part forms the caudal 

 portion of the central nervous system (zpincd cord). Masses of 

 pigment in relation to the sense-vesicle early form the rudiment 



Tries 



no to 



FIG. 734. Later stages in the development of Clavellina. A, approximately median optical 

 section of a larva in which the medullary canal (neuroc<le) has become enclosed throughout, 

 communicating with the exterior only by the neuropore at the anterior end and with the 

 archenteron by the neurenteric canal ; B, larva with a distinct rudiment of the tail and well- 

 formed mesoderm-layer and notochord. Letters as in preceding figure ; in addition, meg. 

 mesoderm. (After Van Beneden and Julin.) 



of the two larval sense-organs, otocyst (or statocyst) and eye. The 

 part behind this presents a thickened wall with a narrow lumen. 

 This is known as the ganglion of the trunk. The rudiment of the 



D 2 



