Introduction to Animal Morphology. 27 



The hardening of the outer surface interferes with 

 the general secretory nature of the surface, and hence 

 in recesses, spheroidal epithelial cells are set apart for 

 this, forming glands. Hardening likewise interferes 

 with the general sensibility of the surface, and hence 

 special processes, tentacles, &c., are developed. Lime 

 salts or chitin may be deposited in the surface cells, 

 forming shells, or in deeper tissues, as in Corals 

 and Echinodermata, or these may ossify, as in the 

 exoskeleton of Vertebrates. 



In simple individuals the only vibrations necessary 

 to be recognised for the purposes of the organism are 

 those caused by material contact or heat. In complex 

 forms, means for the detection of the finer vibrations 

 are provided. For the centralization of perception, 

 the nerve elements are segregated in cells near the 

 end of the body foremost in progression, at the base 

 of special touch organs where such exist, or where 

 the surface is least protected. Around the pharynx 

 these form a single or secondarily double ganglion in 

 each antimere, united to its fellows by a commissural 

 ring, from which nerves pass to the sense organs, to 

 the mouth surface to end in taste cells, and to ciliated 

 cavities possessing the sense of smell. The multipli- 

 cation of metameres is attended with an extension of 

 the nervous system, on the inferior side of the viscera 

 as a twin cord, with ganglia in each metamere. The 

 enlargement of the upper element of the pharyngeal 

 rintf forms a brain, and in Vertebrates this part 

 of the nervous system is prolonged backwards on the 

 upper side of the viscera as a spinal cord. As organs 

 for receiving imprr^Mons and acting on stimuli in- 

 -ain ma^s i- and diffoivntia: 



