i893. OWEN. 29 



period of growth antecedent to the acquisition of the procreative 

 functions, or the adult stage of existence, and that these early 

 chambers are relatively deeper than the succeeding ones. 



In this comprehensive review of the subject Professor Owen 

 states that he " holds by the opinion expressed in his original memoir 

 and in the ' Catalogue of the Fossil Cephalopoda in the Hunterian 

 Museum ' (4th ed,, 1856, p. 29), that they so affect the specific 

 gravity of the active, highly-organised, cephalopodous mollusc, as to 

 enable it with little effort to rise, in the case of the Nautilus, from 

 its habitual position at the bottom of the sea, and in the case of the 

 Spirula, to sink from its more usual zone at or near the surface, 

 by means of the hydrostatic mechanism worked by the muscular 

 forces of the mantle and funnel. The constancy of the siphuncular 

 connection running through all the chambers of the largest and 

 most complex of the polythalamous shells, with the great size and 

 singular complexity of the siphuncle in several extinct species, form 

 the grounds on which I still hold to my original belief in the function 

 of the siphuncle as related to a maintenance of the vitality of the 

 shell." 



In the following year, Professor Owen published the results of his 

 dissections of a perfect specimen of Spirula atistvalis received from 

 Mr. Cuming. The animal was described as almost as devoid of external 

 organs of natation as Nautilus. In both, the direction in which such 

 forces act is retrograde. Nautilus exercises them mainly by virtue of 

 the muscular funnel, through which it forcibly ejects into the sur- 

 rounding water the respiratory current. Spirula superadds to this the 

 ejection of that volume of water upon which the cephalic arms and 

 their basal webs contract after the fashion in which the other 

 Dibranchiata, especially the Octopods, propel themselves backwards. 

 Spirula is superior to Nautilus in the cephalic mechanism. In both 

 instances of multilocular cephalopods, the natatory power is inferior 

 to that of existing Dibranchiata. The distinction, therefore, between 

 Nautilus and Spirula in regard to the shell in its protective relation is 

 relative not absolute : in the one a small proportion of the shell is 

 occasionally " internal," in the other a small proportion is always 

 " external," in both the multilocular shells correspond with the 

 phragmacone of the Belemnite. The tetrabranchiate Orthoceras may 

 be called a representative analogue of the dibranchiate Belemnite, as 

 the tetrabranchiate Ammonite is of the dibranchiate Spirula. The 

 siphon is " ventral " and marginal in both kinds of coiled shells, but 

 it runs along opposite sides of the coil. In Spirula, its position is 

 internal, or ento-marginal ; in ammonites, it is external or " ecto- 

 marginal." 



In 1883 the veteran anatomist published a paper on the " Aspect 

 of the body in Vertebrates and Invertebrates," which formed, we 

 believe, his last contribution to the anatomy of the invertebrated 

 animals. His researches cover a period of fifty years and afford 



