MOLLUSC A. 



211 



middle part of the groove is next converted into a tube, which opens 

 externally in front, and posteriorly communicates with the archente- 



FIG. 116. 



I. CHITON WOSSNESSENSKII. (After Middendorf.) 



II. CHITON DISSECTED to shew o. the mouth; g. the nervous ring ; ao. the aorta; 

 c. the ventricle ; c'. an auricle; br. the left branchiae ; od. oviducts. (After Cuvier.) 



III., IV., V. STAGES OF DEVELOPMENT OF CHITON CINEEEUS. (After Loven.) 



The figure is taken from Huxley. 



ron. The walls of this tube subsequently fuse together, obliterating 

 the lumen, and necessarily causing at the same time the closure of 

 the blastopore. The tube itself becomes thereby converted into a 

 plate of cells on the ventral surface between the epiblast and the 

 hypoblast 1 . 



While the above changes have been taking place the mesoblast 

 has become established. It is derived from the lateral and ventral 

 cells of the hypoblast. 



After the establishment of the germinal layers the further evolu- 

 tion of the larva makes rapid progress. A transverse groove is formed 

 immediately behind the velum, which is especially deep on the 

 ventral surface ; and the stomodseum is formed as an invagination of 

 the anterior wall of the deeper section of the groove. Behind the sto- 

 modgeum the remainder of the ventral surface grows out as a flattened 

 foot. 



1 There is a striking similarity between the changes of the blastopore in Chiton and 

 the formation of the neurenteric canal of Chordata; especially if Kowalevsky is correct 

 in stating that the pedal nerves are developed from the ventral plate. 



142 



