1902.] On the Gastric Organs of Spirula, Nautilus, etc. 231 



viously stated in my memoir of 1894 (vide supra) and I discussed it 

 more fully in 1897 * 



I refer to the matter now, merely to point out that the same deter- 

 mining cause which in the Eutheria calls the " corpus callosum " into 

 being is probably functional in bringing into existence the " aberrant 

 bundle " in the Diprotodontia. 



When the relations of these commissural bundles in the four divergent 

 mammalian groups — Monotremata, Polyprotodontia, Diprotodontia, 

 and Eutheria — are carefully studied we are able to appreciate one 

 — and by no means the least — of the reasons why the Eutheria have 

 attained such a pronounced ascendency over the other three groups. 



Their brain is that which has retained that particular modification 

 of the commissural arrangement which not only furnishes the shortest 

 and most direct path of communication (c) between the two hemi- 

 spheres, but also permits of an unimpeded expansion (which is so freely 

 exercised by the corpus callosum). In the other three groups, in which 

 all the neopallial commissural fibres pass through the ventral commis- 

 sure, the undue expansion of the latter would produce considerable 

 disturbance in the surrounding structures, which in turn would exer- 

 cise a restraining influence upon any marked increase in size in the 

 commissure itself. 



The development of any such commissural mass as the corpus cal- 

 losum of the more highly organised Mammalia in the position occupied 

 by its homologous fibres (fig. 3, a and b) in the Monotremes and 

 Marsupials would cause the most profound disruptions of the corpus 

 striatum, optic thalamus, and the basal region of the brain, and the 

 complete disorganisation of its whole. 



For these various reasons the development of the corpus callosum 

 gives the Eutherian brain a great advantage in the struggle for 

 supremacy, which must have exercised a considerable if not predomi- 

 nant influence in making the Eutheria the highest Mammals. 



" A New Interpretation of the Gastric Organs of Spirula, Nautilus, 

 and the Gastropods. By J. E. S. Moore and W. B. Randles, 

 B.Sc. Communicated by Professor G. B. Howes, F.E.S. 

 Beceived March 17 —Bead April 24, 1902. 



(From the Zoological Laboratory, Royal College of Science, London.) 



Related to the stomachs of some Gastropods and Lamellibranchs 

 there are two conspicuous appendages, the so-called crystalline style-sac 

 and the so-called spiral caecum. 



* "The Origin of the Corpus Callosum," 'Trans. Linn. Soc. of London,' 2nd 

 series, Zoology, vol. 7, part 3, June, 1897, p. 61. 



