CONCLUDING SUMMARY. 647 



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 interestingly, though illusively, 

 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 rarely extends to the ac- 

 quisition by the nascent articulate animal of the locomotive power by 

 superficial vibratile cilia: in the great majority of the province the 

 progeny of the fissiparous primitive germ-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 very great proportion of 

 the Articulate series in reference to their primitive condition as 

 apodal worms. Only in the higher Arachnids, 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 locomotive 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 the 

 Myriapods 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 Molluskf : 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. 



* Von Baer believed that the head completely represented the radiate type, the 

 molluscous structure being seen only on the sac-like part of the body. He 

 remarks, " the Cephalopoda most frequently swim with their heads downwards ; 

 the radiate portion of the body therefore seems to hold and to move itself in 

 accordance with the mode of locomotion prevalent in its type, overcoming the 

 tendency of the molluscous body." — CLXXXIX. p. 760. I have always regarded 

 this supposed combination of the two types as illusory. 



t CLXXXIX. p. 753. 



T T 4 



