ECHINODERMATA 553 



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 vtvy 

 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 peri- 

 haemal canal. Another portion of the system lies in the axial organ 

 and connects 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 alimen- 

 tary canals of other groups. Contractions are said to have been ob- 

 served 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 

 coelomate 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, 

 Ophiuroidea, 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 Holo- 

 thuroidea there is only one gonad, which lies in the "dorsal" inter- 

 radius and has a duct in the dorsal mesentery and a vestigial stolon 

 lying upon the duct, but no rachis. 



