COELOM. 127 



becomes transformed into the perivisceral cavity and its 

 associated spaces (axial sinus, aboral sinus, perihaemal canals) 

 of the adult. 



All the coelomic spaces are lined by an epithelium, which in 

 the case of the water-vascular system and perivisceral cavity 

 is ciliated, and they all contain an albuminous fluid in which 

 amoeboid cells float. 



Nothing of the nature of nephridia is known in the group, 

 but one of the divisions of the splanchnocoelic part of the coelom, 

 viz. that known in the larva as the anterior coelom, has an open- 

 ing to the exterior. This opening is called the primary water 

 pore or the madreporitic pore. The primary water-pore, which is 

 frequently in special relation with a dermal plate called the 

 madreporite, and is always interradial in position, may be a small 

 simple opening or it may be subdivided (Asteroids, Echinoids, 

 many Ophiuroids) into a large number of minute secondary pores, 

 which are more or less closely aggregated together on the madre- 

 porite. Though the generative organs are separate from the 

 coelom in the adult, the primitive germ cells which give rise to 

 them have a coelomic origin. 



We may now proceed to consider the different parts of the 

 coelom in greater detail, and first of all we will treat of the 

 spaces derived from its splanchnocoelic division. These are 

 three in number : the perivisceral cavity, the axial sinus and 

 the perihaemal spaces. 



1. The perivisceral cavity or body cavity proper is always well 

 developed and in relation with the alimentary canal and principal 

 viscera. It is developed from the right and left posterior coeloms 

 of the larva and it never communicates with the exterior except 

 in Crinoids in which the water-pores open into it (see p. 287). 

 It is often traversed by complete or incomplete mesenteries 

 (Holothurians) or by strands of connective tissue, which pass 

 from the body wall to the wall of the alimentary canal. In the 



s / 



brachiate forms the arms always contain prolongations of the 

 perivisceral cavity. As stated above, it usually has a ciliated 

 lining and contains an albuminous corpusculated fluid. 



The Amoebocytes of the coelomic fluids and possibly of other organs 

 play an important part, as was first shown by Durham (op. cit.), in remov- 

 ing foreign bodies from the organism. They act as phagocytes and pass 

 to the exterior by diapedesis through the walls of the papulae on the 

 outside of which they disintegrate. 



