THE CCELOMATE BODY 



temperature in active organs by conveying heat away to the sur- 

 face or to less active regions of the body. This was probably 

 very important in the large extinct reptiles (unless, as some people 

 think, these creatures must have had a proper regulating 



mechanism). 



Although blood seems to be necessary for a large animal, its 

 possession brings dangers. A fluid under pressure is easily lost 

 if the vessel which contains it is pierced, and a medium which 

 takes food and oxygen to the body cells is an ideal environment 

 for parasites, so that it is not surprising that devices have been 

 developed to overcome these dangers ; they are not functions of 

 blood, since if blood did not exist they would be unnecessary, 

 but they are important properties of it. It is not the function of 

 a motor car to have brakes, but they are necessary in order that 

 it can safely perform its function of transporting people from 

 place to place. Excessive loss of blood by bleeding is prevented 

 because on exposure to air blood forms a clot ; parasites are 

 attacked by phagocytes, and the poisons which they produce 

 are neutralised by antitoxins (p. 151). 



Many of the functions of blood may be carried out, often in 

 a less efhcient way, by the coelomic fluid. It must maintain 

 something of a constant environment ; it carries excretory 

 products and contains phagocytes ; and in the earthworm it 

 transmits the pressures produced by the muscles so that burrowing 

 is possible. 



SEGMENTATION 



A division into parts, or segments in a loose and general sense, 

 is common in animals. The egg is sometimes said to segment when 

 it divides into cells or blastomeres ; the limbs of arthropods are 

 divided into jointed parts or podomeres ; and the tapeworm 

 grows a linear succession of proglottides, the oldest being farthest 

 from the growing point. In many coelomates there is a different 

 type of repetition of parts, in which the growing point remains 

 posterior. This is known as metameric segmentation, or often 

 simply as segmentation ; and the repeated parts are called 

 segments or metameres. It seems to be dependent on the meso- 

 derm, for the endoderm and ectoderm are never fully divided, 

 although they may secondarily have to follow the pattern imposed 

 on them by the mesoderm, the segmentally arranged embryonic 

 blocks of which are called somites. In an extreme case muscles. 



