1897-98.] Dr Masterman on Archimeric Segmentation. 273 
cedent in animal morphology. The same remark applies to the 
simplest factor, i.e., three. We are, therefore, left with the 
tetraradiate and pentaradiate forms of symmetry for consideration. 
All the lowest forms ( Coelenterata ) present a tetramerous sym- 
metry, both pelagic and sedentary, whilst the Echinodermata are 
the great pentamerous group. Besides being of higher organisa- 
tion than the Coelenterata , these forms give evidence of owing their 
pentamery to a sedentary past and a peculiar hypertrophy of one 
side of the bilateral ancestor. 
As the hypothetical form we are discussing must have been 
a Coelenterate or little above it, ^ve are apparently justified in 
assuming that it was tetramerous. 
As regards the habitat of this organism, its radial symmetry 
indicates either a pelagic or sedentary existence, and there are 
numerous reasons for holding that it was the former. Brooks* 
has insisted upon the importance of the pelagic habitat as the 
dwelling-place of the primitive types, and on this assumption the 
pelagic stages of the littoral fauna acquire a phyletic significance. 
Some time ago f several reasons were adduced for regarding the 
pelagic ontogeny of certain of the fishes to be primitive and of im- 
portant phyletic significance, and the demersal type of ontogeny to 
be secondarily acquired. The same considerations apply to a large 
extent to the invertebrate forms with pelagic larvae. Amongst 
these, perhaps, the most important is that the typical pelagic 
ontogeny is without yolk, a secondary means of nourishment which 
cannot have its phyletic equivalent. As an important factor in the 
recapitulation of phyletic history an egg must, by the nature of the 
case, have no yolk, and must, from its earliest existence, obtain 
its own nutriment, so that, as these conditions are most nearly 
approached in a typical pelagic ontogeny, we are justified in 
regarding the pelagic habitat as primitive. 
Lastly, we are attempting to follow the evolution of the highest 
and most progressive types of living beings, and it is very question- 
able how far it is possible for organisms which have to any con- 
siderable extent adapted themselves to a sedentary existence, to 
again become free, and thereafter attain the highest position in the 
* W. K. Brooks, The Genus Salpa, Baltimore, 
t A. T. M., Natural Science , March 1897. 
