MOLLUSCA. 271 



The great peculiarity of the early larva of the Scaphopoda 

 is the enormous development of the praeoral lobe, which gives 

 room for the development of the ciliated rings. In the pre- 

 sence of a central tuft of cilia, at the anterior extremity, the 

 larva of the Scaphopoda resembles that of the Lamellibranchi- 

 ata, etc. 



The larva of the Polyplacophora resembles that of Lamelli- 

 branchiata in its anterior flagellum, and that of the Scaphopoda 

 in the large development of the praeoral lobe ; but is quite pecu- 

 liar amongst Mollusca in the transverse segmentation of the 

 mantle area. 



The embryo of the Cephalopoda agrees very closely with 

 that of normal Odontophora in the formation of the mantle and 

 (?) of the shell-gland, but is quite exceptional (i) in the almost 

 invariable presence of a more or less developed external yolk- 

 sack, (2) in the absence of a velum, (3) in the absence of a 

 median foot, and in the presence of the arms. 



The presence of a yolk-sack may most conveniently be spoken 

 of in connection with the foot, and we may therefore pass on to 

 the question of the velum. 



The velum is one of the most characteristic embryonic 

 appendages of the Mollusca, and its absence in the Cephalopoda 

 is certainly very striking. By some investigators the arms have 

 been regarded as representing the velum, but considering that 

 they are primitively placed on the posterior and ventral side of 

 the mouth, and that the velum is essentially an organ on the 

 dorsal side of the mouth, this view cannot, in my opinion, be 

 maintained with any plausibility. 



Various views have been put forward with reference to the 

 Cephalopod foot. Huxley's view, which is the one most gene- 

 rally adopted, is given in the following quotation 1 . 



" But that which particularly distinguishes the Cephalopoda 

 " is the form and disposition of the foot. The margins of this 

 " organ are, in fact, produced into eight or more processes termed 

 " arms, or bracJiia ; and its antero-lateral portions have grown 

 " over and united in front of the mouth, which thus comes, 

 " apparently, to be placed in the centre of the pedal disk. More- 



1 The Anatomy of Invertebrated Animals, p. 519. 



