CEPHALOPODA. 369 



development had not long since rendered that once favourite doctrine 

 untenable. 



Thus as we trace the development of the Molluscous animal, we 

 find the application of the term unity of organisation progressively 

 narrowed as development advances : for whilst all Mollusca manifest, 

 at their earliest and most transitory period, a resemblance to the 

 lowest or monadiform zoophytes, only the lowest order of Mollusca in 

 the next stage of development represents the polypes ; and all analogy 

 to the radiated type is afterwards lost, until we reach the summit of 

 the Molluscous series, when we find it illusively, though interestingly, 

 sketched by the crown of locomotive and prehensile organs upon the 

 head of the Cephalopods. 



In the great Articulated branch of the animal kingdom^ there is 

 unity of organisation with the Molluscous series at the earliest periods 

 of development, in so far as the germ divides and subdivides and 

 multiplies itself; but the correspondence does not extend to the 

 acquisition of the locomotive power by superficial vibratile cilia : 

 the progeny of the fissiparous primitive nucleated cell begin at 

 once to arrange themselves into the form of the Vibrio or apodal 

 worm, while those of the Molluscous germ diverge into the polype- 

 form or into a more special type. 



Unity of organisation prevails through a veiy great proportion of 

 the Articulate series in refei'ence to their primitive condition as apodal 

 worms. Only in the Arachnida apparently, the nucleated cells 

 are aggregated under a form more nearly like that of the mature 

 animal, before they are metamorphosed into its several tissues. In lower 

 or more vermiform Condylopods, the rudimental conditions of the lo- 

 comotive appendages, which are retained in the Anellides and the lower 

 Crustaceans, are passed through in the progress of the development of 

 the complex-jointed limbs. In the great series of air-breathing insects, 

 we have seen that the diverging branch of Myriapoda manifests at 

 an early period the prevailing hexapod type, and that all Insects are 

 at first apterous, and acquire the jointed legs before the wings are fully 

 developed. An articulate animal never passes through the form of 

 the Polype, the Acalephe, the Echinoderm, or the Mollusk : it is 

 obedient to the law of unity of organisation only in its monad stage : 

 on quitting this, it manifests the next widest relations of uniformity as 

 a Vibrio or apodal worm ; after which the exact expression of the law 

 must be progressively contracted in its application as the various 

 Articulata progressively diverge to their special types in the acquisition 

 of their mature forms. 



In the proper Radiated series itself we discern the same principle : 

 the radiated type culminates in the Echinoderms ; but the most typical 



B 6 



