308 Mr. J. F. Blake on the 



of far-separated suboesophageal ganglia. The auditory organ 

 arises at the junction of the upper and lower ganglia, but 

 more in relation to the hinder than the front band of the latter. 

 The hinder pair have been called the splanchnic, and the front 

 pair the pedal ; but their position in this case would be ano- 

 malous, and it is the hinder ganglia which chiefly supply the 

 shell-muscles, which, though not homologous, are to a certain 

 extent analogous to the foot. But if the Nautilus leaves us 

 in doubt, a Sepia*, an Ommastrephes'f, or an Argonaut\ is 

 clear. In these there are three pairs of suboesophageal gan- 

 glia. The front pair supply the arms, the middle pair supply 

 the funnel and the auditory organs, the hinder pair supply the 

 viscera. If, then, we are to take any independent guidance 

 from the nerves, the front pair are not pedal, but belong in all 

 the Cephalopoda to organs not developed in the adult Gastro- 

 poda ; the middle pair correspond to those in the latter class 

 called pedal ; and the hinder pair are the splanchnic. 



To what, then, are the arms homologous? Loven, in 1848 

 (' Bidrag till Kannedomenom utwecklingenafMoll. Acephala') 

 called them a persistent velum ; and to this view Grenadier 

 gives his adhesion. There seems, however, at first sight a 

 fundamental objection to this, as Grenadier himself points 

 out. The velum is always developed on what will be the 

 cerebral side of the oesophagus, while the arms of the Cepha- 

 lopod arise at first on the ojyposite side, or where the foot 

 should be, It is no answer to this to say, without proof, that 

 as they are not needed for nutrition they may shift their place, 

 or, because the oesophagus is unpaired, to make light of its 

 relative position. Such a treatment of questions would render 

 homology hopeless. It seems to me the true solution will be 

 found by asking, What is the velum of a Gastropod ? Hux- 

 ley first, then Gegenbaur, and lastly Hay Lankester have 

 shown how these ciliated bands may be traced from one class 

 to another — sometimes in the larva only, and sometimes as an 

 adult organ (see Lankester on Embryology and Classifica- 

 tion, 1877). In the primitive condition they formed a circle 

 round the oesophagus, and as often as not are thrown out into 

 long processes ; with a change in the direction of the intestine 

 their uniformity is broken, and part dies away, while the other 

 part is left, forming a circle surrounding, not the oesophagus, 

 but a portion of the body on one side of it, the foot being on 

 the other. Since then, in the Gastropoda, the intestine 



* See Huxley, Anat. of Invertebrates, p. 526; after Garner, Trans. 

 Linn. Soc. 1836. 



t Hancock, Ann. & Map. Nat. Hist. 1852. 



| Beneden, Mem. Acad. Brussels, vol. xi. 1838. 



