THE CCELOM 189 



function. The fluid no doubt acts as a shock-absorber for the 

 viscera, which hang in it, and the cells may be excretory, as they 

 are in the earthworm. Its walls generally give rise to the ova 

 and spermatozoa, and may help in excretion, as they do in the 

 vertebrate kidney (p. 615). The coelom always has openings to 

 the exterior. In Annelida there are dorsal pores, and in the dogfish 

 abdominal pores ; these seem to be holes of no well-defined 

 homology or purpose, and perhaps act merely as safety-valves, 

 allowing coelomic fluid to escape if the pressure becomes too great. 

 More important are coelomoducts, which are paired tubes, often 

 segmental, which grow out laterally from the walls of the coelom 

 so that they are made entirely of mesoderm. They are the route 

 by which the gametes escape, and may become highly speciahsed 

 genital ducts, and in the vertebrates they form the excretory 

 tubes. In many annelids, such as the earthworm, the nephridia, 

 which are ingrowths from the ectoderm, may pierce the wall of 

 the coelom and form a third type of communication with the 

 exterior. In many polychaetes each nephridium becomes intimately 

 attached to a coelomoduct to form a composite structure called a 

 nephromixium. 



THE HiEMOCCELE 



The other space in the mesoderm is the haemocoele, or the cavity 

 of the blood-vascular system. This is sometimes, as in the earth- 

 worm, directly derived from the spaces between the cells of the 

 early embryo ; in such animals it is a part of the external environ- 

 ment which has come to be surrounded by cells, and into which 

 fluid is secreted and cells wander. Sometimes, however, as in the 

 vertebrates, it is formed by the separation of mesoderm cells along 

 definite lines, some of the cells being left as the blood corpuscles. 

 Whatever its origin it is always developed as a branching system 

 of vessels with definite walls, which in places are muscular and 

 contractile. In the arthropods and molluscs a part of the blood- 

 vascular system is also expanded to surround the other organs 

 much as the coelom does in a vertebrate ; such creatures have the 

 coelom reduced, and although formally coelomate are very 

 different from other such animals. The term haemocoele is 

 sometimes restricted to these organ-containing expansions of the 

 cavity of the blood system. The haemocoele is sometimes called 

 the primary body cavity, on account of its derivation from the 



