XIII 



PHYLUM OHORDATA 



35 



scn-'ir-vcsiclc (s('7is. res.) 'J'lu' jid.sti'i ioi- nai-i'ow part foiins the caudal 

 pctrtion of the central nervous system {■■^jiiiial cord). JMasses of 

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



mes 





ertcL 



Ttoto 



end. 



Fig. 734.— Later stages in the developmeut of ClaveUina. A, approximately median optical 

 section of a larva in which the medullary canal (neurocfele) 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 di.stlnct rudiment of the tail and well- 

 formed mesodcrm-layer .and notochord. Letters as in preceding figure ; in addition, mti. 

 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 



