EXCRETION. VASCULAR SYSTEM. 131 



of the wall of the axial sinus (except in Crinoids, p. 159), and 

 consists of connective tissue and of cells which have grown 

 into it from the genital rudiment of the larva. It is covered 

 towards the axial sinus by the coelomic epithelium. Its walls 

 are folded so that it appears to be penetrated by tubular pro- 

 longations of the epithelium of the axial sinus and of the peri- 

 visceral cavity. Some observers have attributed to the axial 

 organ a lymphatic gland function, and have supposed that it 

 buds out amoeboid cells into the axial sinus, but this is ex- 

 tremely doubtful. In Echinoids it takes up carmine injected into 

 the body cavity (Kowalevsky). As stated above the primitive 

 germ cells grow into it, so that it is connected in the adult with the 

 generative rachis. The axial organ is absent in Holothurians. 



Excretion. Very little is known about excretion in Echino- 

 derms. There do not appear to be any special organs devoted 

 to it. It is possible that there may be some organ in connexion 

 with the water-vascular system or with the axial sinus which 

 is concerned with the elimination of the nitrogenous waste, 

 for these organs open to the exterior by the water-pore but no 

 such organs have been certainly identified. 



The so-called vascular system, which is found in all classes and 

 appears to be specially well developed in the Holothurians and 

 Echinoids, is formed of a peculiarly modified connective tissue 

 in which the fibres are sparse, and which contains intercom- 

 municating spaces without an epithelial lining. The fluid in 

 these spaces does not appear to undergo any definite movement. 

 The real nature of this tissue is doubtful. By some observers 

 it has been regarded as a lymphatic gland, a view which is sug- 

 gested by the appearance, sometimes found, of amoeboid cells 

 being budded off from it. Typically there is a circumoral tract 

 of it with radial prolongations which lie between the radial 

 water- vessel and the radial nerve cord ; an annular aboral 

 tract of it, in which the generative rachis is embedded 

 and which sends extensions to the genital organs ; and in 

 Holothurians and Echinoids a considerable development of it in 

 the mesentery and on the gut wall. An account of its occurrence 

 will be found in the description of the different classes. 



The generative glands * of Echinoderms are peculiar in the 



* G. W. Field, on the Morphology and Physiology of the Echinoderm 

 spermatozoa, Journ. Morph., Boston, 2, 1895, p. 235. 



