CORPUSCLES OF MARINE WORMS. 137 



in the relations obtaining in different groups of animals 

 between the coelom, on the one hand, and the blood 

 vascular system on the other, have diverged along two 

 lines. The common condition is for the blood vessels 

 to be cut off entirely, or almost entirely, from the body 

 cavity. There is no open connexion between the two in 

 vertebrata and most Chostopod worms. There are two 

 modifications of this state, along divergent paths, but 

 leading to one result, the blending of the blood and 

 coelomic fluid in a common liquid. In Arthropods the 

 blood vascular system is so well developed that the body 

 cavity proper, becoming proportionately diminished, is 

 eventually practically obliterated. The veins cease to 

 exist as such, but are swollen out into enormous sinuses, 

 in which the organs of the body may be said to float. 

 The blood alone must do the work of blood and coelomic 

 fluid, and is, no doubt, specially modified for the double 

 task. The state of affairs in the leeches, little of the 

 coelom remaining, and the blood vessels being well 

 developed, represents a halfway house to this stage. 



The alternative case is when a modification took place 

 exactly the opposite to that in the case of Arthropods. 

 I refer to the condition found in certain Polychseta, 

 the Capitellidge, the Glyceridse, and Polycirrus amongst 

 the Terebellidse. In these, the blood vascular system, 

 instead of enlarging, has dwindled, whilst the coelomic 

 fluid has taken on the double function, both that one 

 proper to itself, and also the respiratory function proper 

 to blood. Even in developing Capitellidge — as I was told 

 by Professor Eisig, who is just finishing his studies on 

 the embryology of the group — even transitory blood 

 vessels do not appear, except in a few doubtful instances. 

 This shows how complete has been the curious alteration 

 in the menage of this little organism. For the resulting 



