552 THE INVERTEBRATA 



anterior cavity) ; the water vascular system (" hydrocoele ") is formed 

 by the transformation of the left second cavity (the right second 

 cavity disappearing); the axial sinus is the persistent left anterior 

 cavity, its madreporite being derived from a '* water pore " which puts 

 that cavity into communication with the exterior. The opening of the 

 stone canal into the axial sinus is the remains of the connection be- 

 tween the left anterior cavity and the left second cavity, which latter, 

 as we have seen, becomes the water vascular system. The madreporic 

 vesicle is budded off from the right anterior cavity (the rest of which 

 disappears); in the larva this vesicle pulsates; it probably repre- 

 sents the pericardium of the Enteropneusta, which retains its con- 

 tractile function in the adult (p. 588). 



All echinoderms except the Holothuroidea possess a peculiar 

 structure known as the axial organ, composed of connective and 



coe. 2/ coe. zr coe. 3^ 



pro. , 



coe. 1/ M coe. 3/ 



Fig. 412. A diagram of the arrangement of the coelom in the ideal Dipleurula. 

 From Sedgwick. An. anus; coe. il. left anterior coelom; coe. 2.1. left middle 

 coelom; coe. zr. right middle coelom; coe. 3/. left hinder coelom; coe. 3^. 

 right hinder coelom; M. mouth; n.pt. neural plate on apex of preoral lobe; 

 pro. preoral lobe or prostomium; por. water pores: only the left of these 

 normally appears ; it becomes the madreporite. 



lacunar ("vascular") tissue, with cells derived from the genital rudi- 

 ment, known as the genital stolon. The axial organ adjoins the axial 

 sinus where the latter is present; in the Crinoidea it lies in the axis 

 of the body. Its function is unknown ; it has been regarded as a heart 

 on account of contractions which it is said to perform, and as an 

 organ of excretion because in echinoids it takes up carmine injected 

 into the body cavity. Oi excretion in the echinoderms little is known. 

 It appears to be performed by the wandering out, through the epithe- 

 lia, of amoeboid cells laden with granules of excreta, and possibly by 

 certain other organs, such as the rectal glands of asteroids, but no 

 constant and conspicuous organs subserve it alone. There are no 

 nephridia. 



Respiration is performed through a variety of structures, some of 

 which expose the coelomic fluid to the external water, while others 



