A treatise on zoology . troiiliicd axial canal of stmi, cnntinuoiis with axial orpin. fornud, and with the extensions of these go also extensions of theaxial sinus (Kig. XXI.). The single gonad of the Ilolothurioidea,connected with the dorsal mesentery, aj)i)ears therefore to be thehomologuo of the axial sinus and organ rather than of ;iny one ofthe intei radial gonads of tlu; other classes. The gonads, therefore, ECHINO DERM A—GENERAL DESCRIPTION 25 are of ciulothclijil origin, riicrc ;4)petirs, however, us shown byHamann, to be a inigratioii of the actual sexual cells; and theview of Cuenot
A treatise on zoology . troiiliicd axial canal of stmi, cnntinuoiis with axial orpin. fornud, and with the extensions of these go also extensions of theaxial sinus (Kig. XXI.). The single gonad of the Ilolothurioidea,connected with the dorsal mesentery, aj)i)ears therefore to be thehomologuo of the axial sinus and organ rather than of ;iny one ofthe intei radial gonads of tlu; other classes. The gonads, therefore, ECHINO DERM A—GENERAL DESCRIPTION 25 are of ciulothclijil origin, riicrc ;4)petirs, however, us shown byHamann, to be a inigratioii of the actual sexual cells; and theview of Cuenot that these are primai-ily amoebocytes deiivedfrom the axial organ suggests their possible mesodermic the migration of sexual cells derived from the ectodermin Hydroids. The giowth and mirnite structure of the ovum havebeen descril)ed l)y Crety (1894), of the spermatozoon by Field(1895); both authors Iefer to pieceding literature. No strikingpeculiarity is presented by Echinoderm gonads (cf. Fig. VI. 1).. Fig. XXI. Diagrams showing relations of psPAidhaemal and water-vasecular systems and axial organ inAsUruis rxOjcns (vdliix Chadwick). 2 is x 50. a?)./i, aboral haemal ring; ax, axial organ; ,axial perihaemal canal; &m, buccal membrane ; (/, genital stnmds; gli, absorbent haemal strandsleading from the gut-wall; lii\ circumoral haemal ring ; ip, inner perihaemal canal (cf. ); /, blood lacunae; il/, niadreporite ; n, nerve ring; op, outer perihaemal canal; rv,radial water-vessel; rli, radial haemal strands ; st, stomacli with folded wall; s(.c, stone canal;wr, circumoral water-ring. The sexes are nearly always separate, and fertilisation takes placein the water. The Haemal Systems of Echinoderma are of two types, whichmay coexist, but of which one usually predominates. Neither is atrue vascular system, but each consists of a series of smaller lacunae(spaces without definite walls) or larger sinuses, sometimes appear-ing as closed, but probably alway
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