Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 FOEM OF THE BODY OF ECHINODERMA. 195 latter larval form is seen in the Astero'ida, the former in the Holothuroida. These forms also obtain in the larvas of other Echinoderma, but in the Ophinrida and Echino'ida a number of processes are developed, on to which the band of cilia is continued. In a few cases (where development takes place within the maternal organism) the larval form is passed over, and the Echinoderm appears without this intermediate stage. The resemblance which exists betwe
Elements of comparative anatomy (1878) Elements of comparative anatomy elementsofcompar00gege Year: 1878 FOEM OF THE BODY OF ECHINODERMA. 195 latter larval form is seen in the Astero'ida, the former in the Holothuroida. These forms also obtain in the larvas of other Echinoderma, but in the Ophinrida and Echino'ida a number of processes are developed, on to which the band of cilia is continued. In a few cases (where development takes place within the maternal organism) the larval form is passed over, and the Echinoderm appears without this intermediate stage. The resemblance which exists between the larval form of even very different divisions points to the common origin of the phylum ; which origin, indeed, was from forms which were not of the radiate type. This important point is ignored by those who try to derive the Echinoderma from the Ccelenterata; they regard the Echinoderma as forming an exception, which they arc unable to explain. The rudiments of the Echino- derm body appear round the enteron of the larva. In the Astero'ida five or more parts are budded off from a common rudiment; these are the future ' arms ' or ' rays ' of the starfish (Fig. 95, A). The free end of the ray appears at first to be in- dependent, and the other end remains in connection with the general mass. This corresponds to the anterior part, and the free end of the ray to 'the posterior part of a worm's body. As the rudiment of each arm grows, joints (metameres) appear in it be- tween the base and the tip. There is a certain amount of independent organisation in each arm of a star- fish ; its organs, such as the intestine, nervous, and vascular systems, and also the sexual organs, have exactly the same position as the homo- logous organs of an Annulate worm. If, then, we compare each of the budding arms with a worm-like organism, we must regard the star- fish developed by this process of gemmation as corresponding to a multiple of such organisms; and, further, we must recognise in this
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