Abstract
ABSTRACT Included in the Chordata are groups of widely different organization which have certain common morphological features at some stage of their existence, a notochord, a hollow dorsal nerve-cord, and gill-pouches or gill-clefts. The various subphyla differ in most other morphological characters, and in their physiology, especially in methods of respiration, feeding, digestion, excretion, reproduction, and locomotion. A study of the composition of the blood in members of the aquatic Chordata shows that variations exist in the equilibrium between the internal and external media which seem to be conditioned by the level of organization of different groups. Most of the data on these animals, however, is incomplete, and rather unsatisfactory on grounds which are discussed below in each section.
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