354 THE STUDY OF ANIMAL LIFE CHAP. 



have arisen the first creatures with a " body ' -the 

 multicellular organisms. This was certainly one of the 

 greatest steps in organic evolution. It was not till later 

 that organs began to be differentiated, but tissues had 

 their beginning in forms like Sponges. The first organs 

 are seen in Ccelentera, and there also we can study the 

 complication of individuality which arises by colony- 

 making. Another great step was the differentiation of 

 male and female multicellular organisms, on which, as 

 we have already noticed, the multicellular Protozoon 

 called Volvox sheds a vivid light. 



In most Sponges and Ccelentera the symmetry of the 

 body is radial and that is well suited for sedentary life 

 and random drifting, but it was an eventful step when 

 some worm-types first acquired definite bilateral sym- 

 metry, a right side and a left, a head end and a tail, and 

 above all head-brains. The establishment of a chief 

 motor, sensory, and co-ordinating centre in the anterior 

 end must have had momentous consequences, and animal 

 behaviour must have risen to a new plane. 



The acquisition of a segmented body, paired appen- 

 dages, and a body-cavity by Annelids marked another 

 epoch, and we must not forget the differentiation of sense- 

 organs and cross-striped, quickly contracting muscle, 

 and such an important discovery on the organism's part 

 as haemoglobin the oxygen-capturing pigment of the 

 blood which first appears in Nemertean worms. Remark- 

 able external armature and jointed appendages were 

 among the gains of the Arthropods. 



One of the epoch-making events was the origin of 

 Vertebrate animals, and then followed the gaining of 

 skulls, jaws, paired fins, and bone itself. Amphibians 

 were the first to have fingers and toes, true lungs, a voice, 

 and a mobile tongue. Among Reptiles we first find the 

 important ante-natal robes (or foetal membranes), and 

 the crocodile gained a four-chambered heart. Birds and 

 mammals are the only warm-blooded animals, and show 

 a great heightening of brain-development. In all mammals 

 except a few primitive types there is a placenta binding 

 the unborn young to its mother in intimate symbiosis. 



