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 

 phenomenon the same process of gemmation as that which takes 

 place in other lower animals; for example, in the compound Ascidians. 

 It is a process in which several separate animals are simultaneously 

 budded off ; the process does not go on till these animals are com- 

 pletely separated, but stops in such a way as to keep them connected 

 together as an individual of a higher order. 



It is not hard to see how, owing to the incompleteness of their 

 separation, the products of gemmation are not only connected ex- 

 ternally, but have a certain number of internal organs connected, 

 and so permanently common to the whole organism. 



o 2 



Fig. 95. Larva of an Ophiurid 

 (Pluteus form), a Rudiment of the 

 Echinoderm with budding arms. 

 d d 1 e 1 Processes of the larval body 

 with their framework of lattice-like 

 rods (after J. Muller). 



