Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 extends on to the arms, and gives passage to the tentacula-like feet. In some the fixed condition obtains in the young only, and later on in life the arm-bearing body breaks off from its stalk (Antedon, Comatula). § 161. The other series of modifications of the form of the body leads to the Echinoida. The arms have altogether disappeared as indepen- dent parts. In the more or less cone-shaped body of the true Sea- Urchins (Desmosticha), the ambulacra extend over the greater part of the sur
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 extends on to the arms, and gives passage to the tentacula-like feet. In some the fixed condition obtains in the young only, and later on in life the arm-bearing body breaks off from its stalk (Antedon, Comatula). § 161. The other series of modifications of the form of the body leads to the Echinoida. The arms have altogether disappeared as indepen- dent parts. In the more or less cone-shaped body of the true Sea- Urchins (Desmosticha), the ambulacra extend over the greater part of the surface. The ambulacral tracts form five bands, which extend from the oral (Fig. 99, A o) to the opposite pole (B a); these are. separated by as many tracts devoid of suckers (Interambulacra). The aboral polar area (apical jj pole) is occupied by the grea tl y- diminishe d ant ambu- lacral surface. The distribu- tion of ambulacral (oral) and antambulacral (aboral) sur- faces of the body, which is pretty equal in the Asteroi'da, is thus completely altered in the Sea-Urchins, for in them the former is much larger than the latter. If we imagine a Starfish-form, in which the arms have completely passed into the general body (cf. Fig. 96, c), then the decrease in size of the antambulacral surface, and the consequent increase in that of the ambulacral, will give us the Urchin-form. This arrangement is modified in the Petalosticha, partly by a change in the relations of the mouth and anus, partly by modifica- tion of the ambulacral tracts. The diminution in size of these tracts is of importance. They form a five-leaved rosette on the dorsal surface, and from these leaf-ends signs of a continuation of the tracts as far as the mouth can be still followed out in the Olypeastridas. The indications of the development of the Echinoderm body from a multiple of individuals are less apparent in the Holothuroi'da than in the Echinoida. But the sausage-shaped body can be derived from the arrangeme
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