ECHINODERMA 293 



and shortened by its muscles. The ampulla is a reservoir connected with 

 the ambulacrum and projecting into the body cavity. In locomotion the 

 animal extends its ambulacra, anchors them by the sucking disc at the 

 tips, and then pulls the body along by contraction of the ambulacral 

 muscles. In the sessile crinoids and the ophiuroids (which move by their 

 snake-like arms) the ambulacra lack suckers and ampulla?, and are not 

 locomotor but tactile in function. So among the holothurians and sea 

 urchins the ambulacra are often replaced by tentacles. Frequently each 

 radial canal ends in an unpaired tentacle with olfactory functions. 



The arrangement of the ambulacral system influences that of other 

 organs. Alongside the stone canal is an elongate organ formerly called the 'heart,' 

 but now regarded as a lymphoid gland or a collection of excretory lymphoid cells 

 (ovoid gland, paraxon gland, septal organ}. Ring and radial canals are accom- 

 panied by corresponding blood canals, with which are often associated two 

 vessels to the alimentary canal. The blood system is surrounded by a peri- 

 haemal space, the ovoid gland and stone canal by the axial sinus, which in the 

 starfishes and urchins passes into an ampulla close beneath the madreporite; 

 this in turn connects with the lumen of the stone canal and also with the exterior 

 through the pores of the madreporite. When the axial sinus is lacking (crinoids, 

 holothurians) the stone canal may open into the body cavity, water entering this 

 in the crinoids by pores in the theca. There is a nerve ring and radial nerve, 

 frequently in the ectoderm, to which may be added an 'apical' nervous system. 



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

 in the animal, the radii; between them come the inter radii. 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. Echinoderms are rarely hermaphroditic. 

 The gonads are supported in the spacious coelom by special bands, while 

 mesenteries support the alimentary tract and its derivatives. 



The five gonads develop from a single anlage which is genetically con- 

 nected with the lymphoid gland. Except in crinoids and holothurians this anlage 

 becomes modified into a perianal ring (rhachis} from which the gonads bud. 

 The so-called blood-vessels hardly deserve the name, since they are fibrous cords 

 with lacunar spaces. The perihagmal space, like the axial sinus and the space, 

 around the gonads and the genital rhachis, are derived from the coelom. 



Respiratory organs are represented by very various structures: branchicc, 

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

 oidea, or on the aboral surface, as in the Asteroidea, thebursce of the Ophiuroidea, 

 the branchial trees of the Holothoroidea and the various parts of the ambulacral 

 system. 



The Echinoderma are exclusively marine, occurring even in the deepest seas. 

 Many groups, like the crinoids, are largely bathybial, others frequent rocky 

 coasts. At the period of reproduction they pass their sexual cells into the sea, 

 where fertilization occurs. In some, however, the young are carried about in 

 brood cases until the earlier developmental stages are past. 



