628 



THE INVERTEBRATA 



code") 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 between 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 represents the pericardium of the Hemichorda, which 

 retains its contractile function in the adult (pp. 667, 668). 



All echinoderms except the Holothuroidea possess a peculiar 

 structure known as the axial organ, composed of connective and 

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



coe. 2/ coe. ir 



por.^^.^ 



pro. 



n.pU ' 



"^An. 



coe. 1 1 M 



coe. 3/ 



Fig. 437. A diagram of the arrangement of the coelom in the ideal Di- 

 pleurula. From Sedgwick. An. anus; coe. il. left anterior coelom; coe. 2/. 

 left middle coelom ; coe. 7.r. right middle coelom ; coe. 3/. left hinder coelom ; 

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

 lobe ; por. water pores (only the left of these normally appears ; it becomes 

 the madreporite); pro. preoral lobe or prostomium. 



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 

 for the lacunar system 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. Of excretion in the echinoderms 

 little is known. It appears to be performed by the wandering out, 

 through the walls of the gills, of amoeboid cells laden with granules 

 of excreta, by the organs of respiration, and by the intestine, but no 

 constant and conspicuous organs subserve it alone. There are no 

 nephridia. The nitrogenous excreta consist largely of ammonia 

 compounds and contain practically no urates. 



Respiration is performed through a variety of structures, some of 

 which expose the coelomic fluid to the external water, Vv^hile others 



