562 INVERTEBKATA CHAP. 



derinata on the one hand and in the Trochophore larvae on the other, 

 there seems to have been a very great difference in the early develop- 

 ment of these forms. The larvae of modern Ctenophora, and Annelida, 

 and Mollusca agree in exhibiting in their development the very 

 early segregation of organ-forming substances in distinct blastomeres, 

 so that by the time the 4-cell stage is reached a single blastomere 

 will only produce a quarter embryo. In a word, they exhibit 

 determinate development. In the eggs of Echinodermata, on the 

 contrary, the separation of organ-forming substances from one 

 another takes place at a much later period ; indeed for a considerable 

 period, in fact until these eggs have attained the blastula stage, 

 any sufficiently large fragment of the developing egg will regenerate 

 the whole. That is to say, the development is indeterminate. It 

 is quite possible that this difference in the character of the early 

 development distinguished the two ancestors from one another 

 before anything like modern Ctenophora, Annelida, Mollusca, or 

 Echinodermata had developed. 



We are now faced with the problem as to what interpretation 

 we are to place on the extraordinary metamorphosis of Echino- 

 dermata. In searching for this interpretation we must recollect that 

 a sudden metamorphosis, like that of Echinoidea, in which all the 

 adult organs are formed under a veil of larval skin and suddenly 

 break forth into activity, is no safe guide to us. In reconstructing 

 ancestral history it is an indispensable condition that there should 

 be continuous and gradual change of function and structure. No 

 animal ever went to bed, so to speak, with one set of habits and 

 woke up in the morning with another. 



Between, however, the larval free-swimming life and the adult 

 slow-moving one, there intervenes, in the case of Asteroidea and 

 Crinoidea, a stage when the animal is fixed by the apex of the 

 prae-oral lobe which has become converted into a stalk. This fixed 

 stage suggests a reason for the radial development of organs which is 

 so marked a feature in the adult Echinoderm. Fixed animals often 

 develop their external food-catching organs in a radial manner, so 

 as to sweep the whole neighbourhood for food. But the radial 

 arrangement of tentacles, as we learn from Coelenterata and Annelida 

 and Polyzoa, is quite consistent with the retention of bilateral 

 symmetry. It can only be described as an idiosyncrasy of Echinodernis 

 that bilateral symmetry is unstable, and that, therefore, radial 

 symmetry was arrived at by the overgrowth of the organs of the 

 left side and the partial suppression of those of the right side. To 

 this day the first sign of uuhealthiness in an Echinoderm larva is 

 an unequal development of the larval arms. 



We may assume, then, that the ancestral Dipleurula took first to 

 holding on with its prae-oral lobe for short periods, a habit which 

 became permanent ; and that then the tendency to inequality 

 between the two hydrocoeles, which we may suppose to have been 

 latent in the stock, became a positive advantage as a short cut to 



