268 LECTUKE XX. 



from any of the Vertebrate forms of animal life. How vast the 

 hiatus which separates the worm from the apodal H»h, the crab from 

 the tortoise, and the flying insect from the bird or bat ! He soon 

 attains the conviction that there is no regular and uninterrupted 

 ascent in the scale of organisation, as Bonnet fancied ; no single and 

 continuous chain of beings, as was sung by Pope. 



Not even with the insight which we now command into the living 

 forms that peopled this planet during past and remote epochs of its 

 history, can we supply all the hiatuses which exist, and connect 

 together in a linear series the existing and extinct members of the 

 Animal Kingdom. But we can discern that many connecting links 

 in partial series have perished ; and we know that the broken hypo- 

 thetical chain of being nevertheless continues to flourish and to ade- 

 quately fulfil its appointed office in maintaining the balance of the 

 conflicting influences of increase and decay, and the general well- 

 being and progress of organic life upon the present surface of the 

 earth. 



If we would make a closer approach to the vertebrate type of 

 organisation, we must retrace our steps, and, again returning to the 

 Radiata, ascend by another and very different series of animals, from 

 those which have last occupied our attention. 



In the Articulata the advance is most conspicuous in the organs 

 peculiar to animal life, and was manifested in the powers of locomo- 

 tion, and in the instincts, which are so various and wonderful in th 

 insect class. 



In the Mollusca the organising energies seem to have been ex- 

 pended chiefly in the perfection of the vegetal series of organs, or 

 those concerned in the immediate preservation of the individual and 

 the species. 



The Mollusca are so called on account of the soft unjointed nature 

 of their external integument. The scattered centres, or Heterogan- 

 gliate type of their nervous system, is often accompanied with an 

 unsymmetrical form of the entire body ; which, in compensation for 

 the low condition of the perceptive energies, is protected in most of 

 the species by one or more dense calcareous plates, called shells. 



The nervous system, as has been explained in the introductory 

 lecture, consists of a medullary collar, surrounding the oesophagus, 

 and communicating with more or fewer ganglions near the oesophagus, 

 or dispersed, usually below the alimentary canal, in other parts of the 

 body. 



In a large proportion of the lower organised Mollusca there is no 

 head, and no nervous centre is needed above the gullet for the recep- 

 tion of the impressions received by special organs of sense. The 



