CHAPTEE IX. 



THE COELOMATA. 



THE coelom, the possession of which characterizes all the remaining 

 groups of the animal kingdom, is an organ of the greatest importance, 

 and one the real nature of which has only recently been appreciated 

 by anatomists. 



Formerly the word coelom was used as synonymous with body- 

 cavity or perivisceral cavity, and no distinction was recognized 

 between the body-cavity of the Arthropoda and the same structure 

 in such forms as the Vertebrata. In fact, there are works on 

 Zoology now in use in which the term coelom is applied alike to 

 the blood-containing space of the Arthropods, and to the body-cavity 

 of Annelids and Vertebrates, which is free of blood, and into which 

 the generative ducts and kidney tubes open and the generative cells 

 are dehisced. "We now know that the organ called coelom, so far 

 from being necessarily or primarily a body-cavity, may in some cases 

 have nothing of that mechanical relation to the viscera which is 

 implied in the conception of a perivisceral space. It is true that 

 in the majority of cases a portion of the coeloni does enlarge, acquire 

 a thin smooth Avail, and enter into a relation with some of the more 

 important viscera, a relation the purpose of which is apparently to 

 enable the functional movements of the internal organs to take place 

 with the least amount of friction and of resistance from surrounding 

 structures. But there is one group, the Arthropoda, in which the 

 coelom, though present and discharging important functions, develops 

 no such perivisceral portion, and another, the Mollusca, in which the 

 perivisceral portion is sometimes much limited, not extending to any 

 other viscus than the heart. 



The functions which the coelom discharges in such cases, and the 

 real nature and relations of the organ will be dealt with more fully 

 in the general part of this work ; here we shall only consider the 

 subject in its main outlines, and not being in a position to place all 



