THE CEPHALOPODA. 455 



the cavity of the shell which lies nearest its open end, and 

 the rest of the cavity is divided into chambers, which contain 

 air, by transverse septa. The septa are perforated, and a pro- 

 longation of the mantle — the siphuncle — is continued through 

 the series of perforations, as far as the apical chamber of the 

 shell. The internal shells of the Cephalopods may have 

 very various forms, and may even be chambered and siphun- 

 culated; but, in this case, the chamber nearest the mouth of 

 the shell is small, and incapable of lodging the viscera. 



Our knowledge of the development of the Cephalopods is 

 confined to that of the DibranchiataS In these, the yelk 

 undergoes partial division, and the blastoderm, formed upon 

 one face of it by the smaller blastomeres, spreads gradually 

 over the whole ovum, inclosing the larger and more slowly- 

 dividing blastomeres. The mantle makes its appearance as 

 an elevated patch in the centre of the blastoderm, while the 

 future arms appear as symmetrically-disposed elevations of 

 the periphery, on each side of the mantle. Between these 

 and the edge of the mantle, two longitudinal ridges mark the 

 rudiments of the epipodia, while the mouth appears in the 

 middle line, in front of the mantle, and the anus, with the 

 rudiments of the gills, behind it. The rest of the blastoderm 

 forms the walls of a vitelline sac, inclosing the larger blasto- 

 meres. 



The pallial surface now gradually becomes more and more 

 convex, the posterior margin of the mantle growing into a 

 free fold, which incloses the pallial chamber and covers over 

 the gills. 



The internal shell is developed in a sac formed by an in- 

 volution of the ectoderm of the mantle. The epipodia unite 

 behind, and give rise to the funnel, while the anterolateral 

 portions of the foot grow over the mouth, and thus gradually 

 force the latter to take up a position in the centre of the neu- 

 ral face, instead of in front of it. The yelk-sac gradually 

 diminishes, and the contained blastomeres are finally taken 

 into the interior of the visceral sac, into which the alimentary 

 canal is gradually drawn. 



The Cephalopoda are divided into two very distinct 

 groups, the Tetrabranchiata and the Dibranchiata. 



The Tetrabranchiata possess an external chambered si- 



1 Kolliker, " Entwickelungsgeschichte der Cephalopoden," 1841. Gre- 

 nacher, u Zur Entwickelungsgeschichte der Cephalopoden " (Zeitschrift fur 

 wiss. Zoologie, 1876). Lankester, "Observations on the Cephalopoda" 

 {Quarterly Journal of Micr. Science, 1875). 



