A treatise on zoology . Via. xn. Dorsal, /.»?. uhnral, vinw ofAstfrina gilihnstt, tMti days olil(aftorLiuiwi^j). MhovvM pn-Driilloll* (/.) in)w oil oriil Midf ;rinliiiitMits of tl vi- uriiis innrknil\>y tlvt) trriiiliinl |)(7); al-turiiafinv; witli tlust- arc llvfImMftlM (/•); RM*I lit Im ;alWi<l i|oim)-c»Mitral ; M,in«di«j»orit«. xi»<. ECHINODERMA—GENERAL DESCRIPTION 15 of the Pentadcea^ together with epithelial nerves on the floor ofthe arm-grooves, as in Pelmatozoa. The Ophiuroids are, asexplained under Stelleroidea, scarcely to be distinguished fromAsteroids. Whethe
A treatise on zoology . Via. xn. Dorsal, /.»?. uhnral, vinw ofAstfrina gilihnstt, tMti days olil(aftorLiuiwi^j). MhovvM pn-Driilloll* (/.) in)w oil oriil Midf ;rinliiiitMits of tl vi- uriiis innrknil\>y tlvt) trriiiliinl |)(7); al-turiiafinv; witli tlust- arc llvfImMftlM (/•); RM*I lit Im ;alWi<l i|oim)-c»Mitral ; M,in«di«j»orit«. xi»<. ECHINODERMA—GENERAL DESCRIPTION 15 of the Pentadcea^ together with epithelial nerves on the floor ofthe arm-grooves, as in Pelmatozoa. The Ophiuroids are, asexplained under Stelleroidea, scarcely to be distinguished fromAsteroids. Whether they branched ofl at an earlier date or no isuncertain ; at any rate, they have progressed farther from thePciitactaa type, in so far as the radial nerves have sunk belowthe surface and are covered by epineural canals, which probablyrepresent closed food-grooves (Fig. XIII. 1 and 2).. 2 ^^ Fig. XIII. Sections across ambulacra of—1, Asteroid ; 2, Ophiuroid ; 3, Echinoid ; 4, , ambulacral ossicle; , ampulla ; }>, radial blood-vessel; cm, circular muscles ; ^ventral scute ; e, radial epineural canal; hn, longitudinal muscles ; , lateral scute ; m, muscles ;11, radial nerve of supertlcial oral system ; n2, radial nerves of deeper oral system ; p, podium ;ph, pseudhaemal canal; , vertebral ossicle ; w, radial canal of the water-vascular system. The development of Echinoidea has been studied by J. Miiller(1852), Agassiz (1864), Metschnikoff (1869), Bury (1889), andmany others. The results are summarised by Theel in his admir-able account of the development of Echinocyamus pusillus (1892).Up to the stage corresponding to the Dipleurula no importantdivergences are manifest. The peculiarities of the ensuing meta-morphosis appear due to the extreme development of a free-swimming Pluteus (Fig. XIV.). At an early stage there is aninvagination
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Keywords: ., bookcentury1900, bookdecade1900, bookpublishe, booksubjectzoology
