254 



POLYPLACOPHORA. 



folds at first merely form the side of a groove or imperfect tube 

 (fig. 1 1 3 C and 1 24 ff.), but soon the free edges unite and so give 

 rise to a perfect tube, the primitive origin of which by the coal- 

 escence of two halves would not be suspected. In Nautilus the 

 two halves remain permanently separate but overlap each other, 

 so as to form a functional tube. 



Polyplacophora. The external characters of the embryo of 

 Chiton have long been known through the classical observations 

 of Loven (No. 285), while the formation of the layers and the in- 

 ternal phenomena of development have recently been elucidated 

 by Kowalevsky (No. 284). The eggs are laid in April, May, 

 and June, and are enclosed in a kind of chorion with calcareous 



FIG. 1 1 6. 



I. CHITON WOSSNESSENSKII. (After Middendorf.) 



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

 aorta ; c. the ventricle ; S. an auricle ; br. the left branchiae ; od. oviducts. (After 

 Cuvier.) 



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

 The figure is taken from Huxley. 



protuberances. The segmentation remains regular till sixty- 

 four segments are formed. The cells composing the formative 



