15 



And with respect to the principles on which the organs 

 act, they will he found to comprise three, viz. : — 



1st. Simple mechanical re-action; 



2d. Adhesion ; and 



3d. A union of hoth re-action and adhesion. 

 In the application of these principles, Nature so arranges 

 them that the central groups present most distinctly the 

 essential characters of the type. 



The author commenced with the " Porifera," as the 

 lowest of the animal kingdom, and of these, selected the 

 common sponge as a type. The vital phenomena of the 

 adult sponge were descrihed, and its claims to he admitted 

 to the animal kingdom stated. Tins creature has the power 

 of locomotion in the first stage of its separate existence only. 

 The migratory movements of tins sponge-gemmule are, it 

 was stated, effected by means of cilia thickly planted at its 

 upper extremity, and the cilium is thus shown to be the first 

 organ of locomotion which Nature has constructed, so far as 

 yet discovered. What may be the precise structure of this 

 organ, and how attached and worked, whether by means of 

 an apparatus of muscles, like the 'spine of the Echinus, or 

 formed by the involution of the parieties to which it is 

 adherent, like the velli of the intestinal tubes of the higher 

 animals — and by what process of excision it is cast off, so 

 soon as the creature has found a suitable habitat, and no 

 longer requires its aid, the microscope has, hitherto, failed 

 to disclose. The employment of this primitive organ of 

 locomotion is not, however, confined to the humble creature 

 for which it was constructed, but may be traced through the 

 various ascending grades of animal development even up to 

 man. 



The Animals next mentioned, on account of their low 

 organization and simplicity of structure, were the Polypi, 

 a race possessed of much more vigorous animal powers than 

 the preceding. Although the organs of locomotion make 



