ECHINODERMATA 629 



carry the water into the body and expose it to the fluid in the coelom. 

 To the first class belong the podia, and the "gills" of asteroids and 

 echinoids; to the second belong the "genital bursae" of ophiuroids 

 and the respiratory trees of holothurians. 



The vascular system of other animals is represented in the Echino- 

 dermata by a system of strands of a peculiar lacunar tissue, containing 

 intercommunicating spaces which have no epithelioid lining. Ulti- 

 mately, this system is of the same nature as the blood vessels (haemo- 

 coele) of other animals, since both are systems of spaces derived from 

 the blastocoele and filled by a fluid matrix containing free cells ; but 

 in appearance, and probably in the mode of its functioning, it is very 

 different. A ring of lacunar tissue surrounds the mouth, lying in or 

 immediately above the perihaemal ring and giving off in each radius 

 a strand or "vessel" which similarly lies above the radial perihaemal 

 canal. Another portion of the system lies in the axial organ and con- 

 nects the oral ring with an aboral ring, which accompanies the genital 

 rachis (see below) and sends strands to the gonads. In the Echinoidea 

 and Holothuroidea two strong "dorsal" and "ventral" vessels from 

 the oral ring accompany the alimentary canal, running on opposite 

 sides of that organ and giving off a plexus of branches which ramify 

 on it, and in holothurians also in a perforated fold of the peritoneum. 

 A "vascular" plexus is also present on the alimentary canals of other 

 groups. Contractions are said to have been observed in parts of the 

 system, but it is very doubtful whether anything in the nature of a 

 regular circulation takes place in it, though it probably maintains 

 communication by diffusion between various parts of the body. 



With rare exceptions, the sexes of echinoderms are separate. The 

 genital organs are remarkable for their simplicity. They possess neither 

 organs of copulation, nor accessory glands, nor receptacles for the 

 retention of ova, nor a reservoir for the storage of sperm in either sex, 

 and they discharge direct to the exterior and not, as is usual in coelo- 

 mate animals, through the coelom or through ducts proper to that 

 cavity. Nevertheless they arise in ontogeny from the coelomic wall. 

 The genital system consists, except in the Holothuroidea, of the 

 genital stolon, a collection of cells in the axial organ ; the genital rachis, 

 a ring connected with the stolon (aborally in the Asteroidea, Ophiu- 

 roidea, and Echinoidea, orally in the Crinoidea); the gonads proper, 

 which are sacs or tubes, often branched, borne upon long or short 

 branches of the rachis and varying in number in the different groups ; 

 and the short ducts, lacking in the Crinoidea. In the Holothuroidea 

 there is only one gonad, which lies in the "dorsal" interradius and 

 has a duct in the dorsal mesentery and a vestigial stolon lying upon 

 the duct, but no rachis. 



The nervous system consists of networks of fibrils and nerve cells 



