FIVE GREAT ANIMAL GROUPS. 125 



this chain is on one side of the intestine, opposite to the heart (h), 

 but yet one of the ganglions, that of the sense of taste, (g,) one of 

 the chief senses, is placed on the opposite side of the intestine, 

 and next the heart. The position of the internal skeleton is rep- 

 resented by a line (ch, ch } , cA 2 ) running between the main nerve- 

 trunk (en, we 1 ) and the intestine (m, a). 



I know some naturalists would object that the Mollusca, 

 Articulata, and Zoophyta cannot correspond, in the nervous 

 system, with the Vertebrata, because the first three have a ner- 

 vous ring or collar, (c,) and the brain is on the opposite side 

 to the nervous cord ; but this, upon careful comparison, will be 

 found to be not true ; for, in the first place, the Vertebrata do 

 have as distinct a nervous collar (figs. 63, 64, c) as some Mol- 

 lusca, and secondly, one of the principal senses, taste, is centred 

 in a ganglion (") which is reached from the main nerve by this 

 collar of nerve-threads which passes around the intestine exactly 

 in the same way as in Mollusca and Articulata. Moreover, in 

 regard to the position of the so-called brain in Mollusca and 

 Articulata not corresponding to that in Vertebrata, I would say 

 that in the Mollusca the position of the sense-ganglions varies, 

 but in the highest of them, the Cuttle-fishes, the ganglion for the 

 eye holds almost the same relation to the main trunk as does 

 the corresponding organ in Vertebrata ; and in the Articulata, 

 the position of the eye-ganglion also varies, for although among 

 Insects it is on the opposite side from the main nerve, yet among 

 a large group of Worms, the Nemertians, the ganglion from 

 which the optic nerve springs is on the same side as in the Ver- 

 tebrates. Between these two extremes of position there are all 

 grades. 







As to the objection raised by those who claim that the verte- 

 bral column, or bone of the back, is the distinguishing feature of 

 Vertebrata, I would say that when we refer to certain fishes, 

 such as Lamprey eels and the Sturgeons, we find the backbone 

 represented by a mere gristly cord, and in other fishes (Myxine, 

 Ammocoetes) a cord of interlaced fibres in place of vertebra. 

 Yet in several respects these fishes are far more highly organized 



