254 ANIMAL PHYSIOLOGY. [LESSON 77. 



separated by an interval of space, and connected by a nervous chord 

 which passes behind the oesophagus. 



1152. In the Articulata we found the alimentary canal invariably 

 transmitted through a nervous ring which surrounded it ; thence the 

 remaining ganglia were periodically developed at stated intervals 

 upon a double nervous chord, extending in the direction of the long 

 axis of the body. 



1153. In the Mollusca the ganglia are always connected by a sin- 

 gle, not a double, chord ; moreover, the number and disposition of all 

 except the cephalic ganglions, is extremely variable, and of these 

 only one is met with in the lowest animals of the class. 



1154. Again : in the Articulata, the advance in the development 

 of the nervous system is most conspicuous in the organs peculiar to 

 animal life, and is chiefly manifested in the powers of locomotion, 

 and in the instincts which are so various and wonderful in the class 

 of insects. 



1155. In the Mollusca, on the contrary, the organizing energies 

 seem to have been chiefly expended in the perfection of the nutri- 

 mental series of organs, and of those concerned in the immediate 

 preservation of the individual and of the species. 



1156. The Mollusca are so called on account of the uniformly 

 soft, unjointed nature of their external integument. 



1157. In a large number of the lower organized Mollusca, there 

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

 reception of the impressions received by special organs of sense. All 

 other Mollusca are provided with a head, which generally supports 

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



1158. Amongst the lowest of the molluscous animals, is the class 

 Tunicata ; in these creatures the nervous system is more rudimen- 

 tary than we have described it ; only one ganglion has yet been dis- 

 covered, from which the filamentous nervous system is distributed 

 throughout the body. 



In Cynthia pupa (Fig. 253) the nervous system consists of a sin- 

 gle ganglion (n) connected to a nervous chord which surrounds the 

 respiratory orifice (a). Fine nervous twigs are distributed to the 

 large vibratile organs, and a chord (d), of more than ordinary di- 

 mensions, commences at e, and passes down, without distributing 

 branches, to the bottom of the sac, where it is lost. 



1159. The second order of the class Tunicata, includes the Sal- 

 pians, which float in the open sea, and are characterized by their 

 transparent elastic outer tunic, which is elongated, compressed, and 

 open at both extremities. 



