470 LECTURE XX. 



that peopled this planet during past and remote epochs of its history, 

 can supply all the actual hiatuses, 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 hypothetical chain of Nature 

 nevertheless continues to flourish and to adequately fulfil its ap- 

 pointed ofiice 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 occupied our attention in the preceding Lectures. 



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 the 

 insect class. 



In the Mollusca the developmental 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 of the nervous 

 system, disposed according to the Heterogangliate type of that domi- 

 nant system of organs, 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. 



All Mollusca have a complete alimentary canal, with mouth, 

 stomach, intestine, and vent ; and they are provided with circulating 

 and respiratory organs. 



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 brain ; no nervous centre being needed above the gullet 

 for the reception of the impressions received by special organs of 

 sense. The inlet for the food is simply a pharynx or beginning of 

 the oesophagus, without jaws, tongue, or mouth properly so called. 

 All other Mollusca are provided with a head, which generally sup- 

 ports feelers or soft tentacula, eyes, and a mouth armed with jaws. 



