XVII. BRANCH XII. — CHORDATA (chord, cord) 



If we recall, thoughtfully, all the animals thus far studied, 

 we shall find that they are conspicuous for the lack of one 

 thing. That is, that they have no backbone or anything to 

 take the place of a backbone; hence they are called inverte- 

 brates (in, without; vertebra, joint). All the remaining 

 members of the animal kingdom are conspicuous from the 

 fact that they have a spinal column or some structure that 

 takes the place of it at some period of their life. Those 

 that possess a real backbone, as most fishes, all birds, and 

 mammals, are known as vertebrates. Some of the animals 

 with which we are yet to become acquainted have no 

 spinal column, but they do have a structure that takes 

 the place of it. This is a soft, flexible rod, or cord, that 

 tapers to both ends and Ues along the back, where the 

 backbone lies in vertebrates. Again, a very few animals 

 — some sea squirts — possess this cord only in their youngest 

 stages, losing it entirely when full grown. Finally, all 

 vertebrates, as fish, birds, and mammals, possess this cord 

 in their embryonic stages, but its place, in most, is taken 

 later by a spinal column. Hence all animals, not included 

 in the eleven branches already discussed, possess, either in 

 their youngest stages or throughout life, a soft, flexible 

 cord, or rod, known as a notochord, and consequently they 

 are grouped together in one branch called the Chordata. 



Moreover, when any animal so far noted has had a cavity 

 in the body, it has had but one; for example, the hydra, 



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