ECHINODERMA. 331 



orally to a ring canal around the mouth. The ring canal bears 

 usually several (up to five pairs) Polian vesicles, which, with 

 Tiedemann's vesicles of the starfishes, are now regarded as appen- 

 dages which, like lymph glands, produce the leucocytes. From 

 the ring canal radiate five radial canals which give off right and 

 left in pairs the ambulacral canals. These in turn connect with 

 the ambulacra and ampullae, the highly characteristic locomotor 

 organs of the echinoderms. An ambulacrum is a muscular sac 

 which can be distended and lengthened by forcing in fluid from 

 the ambulacral vessels, on the other hand can be retracted and 

 shortened by its muscles. The ampulla is a sac connected with 

 the ambulacrum and projecting into the body cavity. In locomo- 

 tion the animal extends its ambulacra, anchors them by the suck- 

 ing disc at the tips, and then pulls the body along by contraction of 

 the ambulacral muscle. In the sessile crinoids and the ophiuroids 

 (which move by their snake-like arms) the ambulacra are not 

 locomotor but tactile in function, lacking suckers and ampullae. 

 So among the holothurians and sea urchins the ambulacra are in 

 many places replaced by tentacles. Frequently each radial canal 

 ends in an unpaired tentacle with olfactory functions. 



The arrangement of the ambulacral system conditions the 

 arrangement of other organs. Alongside the stone canal is a 

 saccular organ formerly called the ( heart/ but now regarded as a 

 lymphoid gland (ovoid gland, paraxon gland). Ring and radial 

 canals are accompanied by corresponding blood canals, with which 

 are often associated two vessels to the alimentary canal. There is 

 a similar nerve ring and radial nerve, frequently in the ectoderm, 

 to which may be added an enteroccelic or apical nervous system, 

 possibly of peritoneal origin. 



The courses of the radial vessels and nerves mark out five chief 

 lines in the animal, the radii; between them come the secondary 

 radii or interradii. The stone canal, madreporite, and lymphoid 

 gland are interradial in position, as are the gonads, usually five 

 single or five pairs of racemose glands; in some cases but one is 

 present. The gonads are supported in the spacious ccelom by 

 special bands, while mesenteries support the alimentary tract and 

 its derivatives. 



Respiratory organs are represented by very various structures: branchiae, 

 or thin-walled outpushings of the coelom, either around the mouth, as in 

 Echinoidea, or on the aboral surface, as in the Asteroidea, the bursae of the 

 Ophiuroidea, the branchial trees of the Holothuroidea and the various 

 parts of the ambulacral system. 



