ECHINODERMATA—FHYLOGENY 54^ 



Metamorphosis of the Dipleurula Larva. 



The larva develops, through metamorphosis, into the young Echino- 

 derm, which, under its radiate mask, is asymmetrical. The radiate 

 structure is amalgfamated with an asymmetrical structure. 



Here again, we are not altogether without light as to the phylo- 

 genetic significance of this process. We agree with the majority of 

 modern authors in believing that a radiate structure of body arises 

 as a consequence of an attached manner of life. We are, therefore, 

 justified in assuming that the radiate Echinoderm arose from a free- 

 moving racial form, in adaptation to a newly -acquired, attached 

 manner of life. 



All Echinoderms must, therefore, once have been attached 

 animals. 



If now, we wish to ascertain in what special manner attachment 

 took place, we unavoidably turn for an answer to this special question 

 to the Crinoids. These are the only Echinoderms which, in all prob- 

 ability, never again gave up the attached manner of life. That the 

 only Crinoid about wliose ontogeny we know anything, Antedon, is a 

 form which has actually once more become free, i.e. has, as a secondary 

 specialisation, given up the attached manner of life, detracts in no way 

 from the arguments based upon its ontogeny. 



All other Echinoderms whose ontogeny we can investigate have 

 long since given up the attached manner of life, and, with the excep- 

 tion of certain (analogous) cases among the Asteroids (e.g. Asteritm), 

 do not an}- longer pass through an attached larval stage. Hence the 

 methods of development of other Echinoderms, even when simpler 

 than that of the Crinoids, must in comparison with the latter be regarded, 

 phylogenetically, with some suspicion. 



From the developmental history of Antedon then, we learn that the attacliment 

 of the Dipknirula larva of this animal took place by means of the ventral side of 

 the anterior end of the body. In a similar way the Dipleurula larva of Astcrina 

 attaches itself by means of the larval organ which develops anteriorly. 



Authors who have recently attacked this problem assume that attachment took 

 place on the right side ; making this assumption in order to explain the asymmetry 

 wliich follows. To us also this assumption appears necessary, but it should be 

 specially stated that the attachment took place on the right anteriorly. 



When this assumption is made we must further ask : AVhat Avere tlie changes 

 Avhich the attached manner of life induced ? 



It is difficult, with the embryological material we have at present, to obtain an 

 adequate idea of the resulting processes, and only a very tentative explanation can 

 be given. 



Judging by analogy from the modifications which, in other parts of the animal 

 kingdom, result from an attached manner of life, it may be assumed that the arrange- 

 ments for conducting food were the first to become adapted to the new condition of 

 existence. The mouth left its unfavourable position and wandered along the ventral 

 side, first to the left, i.e. to the side which was now uppermost (being opposite to the 

 point of attachment). In this shifting the n-sophagus pushed the median and 



