

- 
: . The other initial number 2n being given the 
successive values 6, 8, etc., or 
n=3,4,... 14, 

: the formula gives, with N=109721-6, all the twelve 
observed members of the series from m=2 to m=13, 
pn eae the first with a deviation of 5, the second 
wi 
in 0-7, and the remaining ten members within 
a fraction, ranging from o-1 to 0-35 A.U. 
The possibility of reducing 4N to N, based on the 
fact that all numbers are even, is interesting, 
especially as it forces itself on us also in the case of 
the Bement! and the principal series of singlets, 
which, though less precisely but again orderly and 
without gaps, are represented by 
raax()an(2), 24.5, 
and = 
p=4N(14-2") =n(Z-2), F793, 45 5) ss 14, 
This reducibility (to one N), if interpreted physically, 
would mean that the helium nucleus attracts each 
of its electrons with only one-half of its total charge, 
as if its lines of force formed two bundles, each 
entirely engaged with one of the two trabants. 
Details concerning these three series and the last- 
mentjoned possibility will be given at the coming 
. Boston meeting of the American Association. 
LupWIK SILBERSTEIN. 
129 Seneca Parkway, Rochester, N.Y., 
December 13. 

Echinoderm Larve and their Bearing on 
Classification. 
May I ask your permission for a short space in 
which to reply to Dr. Mortensen’s letter published in 
Nature of December 16, p. 806, under the title 
“ Echinoderm Larvae and their bearing on Classifica- 
tion.’’ The points which Dr. Mortensen raises are two 
—namely (a) whether the Echinoderm metamorphosis 
is a metagenesis, i.e. an alternation of generations, or 
not, and (b) whether the fixed stage in the life-history 
of Asteroidea is a reminiscence of an ancestral con- 
dition or a secondary modification of development, I 
shall deal with the second point first. Dr. Mortensen 
states: 
(1) That the group Spinulosa among Asteroidea are 
not primitive but modified forms and that the 
illosa are the more primitive group, and that in 
this view certain modern systematists whom he 
quotes agree with him. 
(2) Thatsince the Astropectinide (Paxillosa) do not 
have a Brachiolaria stage in their ontogeny, this stage 
is not primitive and ancestral but secondarily inter- 
calated where it occurs in the development of Spinu- 
losa and Forcipulata. 
I must confess that I am unconvinced by Dr. 
Mortensen’s arguments. In his original work, re- 
viewed by Dr. Bather, he forgot that the Brachiolaria 
larva was found in Spinulosa but referred it to 
Forcipulata only. 
The systematists whom he quotes are neither 
paleontologists nor physiologists but—for the most 
part—students of the external features of preserved 
specimens only. Koehler (one of them) regards Hud- 
sonaster, one of the oldest Asteroids known, as 
“ voisine des Astropectinides,’’ and W. K. Fisher also 
states that “ typical Phanerozonia such as the Astro- 
# are more primitive than the Spinulosa.” 
‘ Now what these specialists are impressed by is the 
phanerozonate ’’ character of the Astropectinide, 
that is, the edging of the arms with a series of broad 
NO. 2776, VOL. 111 | 
] NATURE 
47 
plates termed the ‘“‘ marginals.’’ 1 have always pro- 
tested against regarding this feature as a primitive 
character and in this protest I have the support of 
the best British authority on fossil starfish, W. K. 
Spencer. The fact is, the apparent marginals of these 
ancient starfish are not homologous with the marginals 
of the modern Paxillosa at all but are the adam- 
bulacrals. Reasoning from imperfectly described 
fossils and superficially described modern forms has 
completely misled the older systematists. 
Ludwig, whom Dr. Mortensen quotes, was a worthy 
pcos in the knowledge of Echinoderms, but he 
elongs in all his thoughts and views to another epoch. 
His classification, for example, of the Holothuroidea 
into Actinopoda and Paractinopoda has been com- 
pletely disposed of by modern embryological research. 
In my letter of a year ago, I gave physiological and 
anatomical reasons for regarding the Astropectinide 
as Asteroids secondarily modified for a life on sand. 
I can only express the doubt whether Dr. Mortensen 
could have regarded the Astropectinids as primitive 
if he had ever thoroughly dissected one. 
With regard to the homology of the stalks of the 
Bracholaria larva of the Asteroid and the Penta- 
crinoid larva of Antedon, I should like to reiterate 
the following facts : 
(1) The larve are, broadly speaking, comparable ; 
in both there is a long preoral lobe, a ventral stomo- 
dum, right and left posterior coelomic sacs. F 
(2) In both forms there is a fixing (‘‘ sucking "’) disc 
formed at precisely the same spot, and in both the 
preoral lobe is converted into a stalk. p 
Is it not infinitely more probable that the precisely 
similar stage of fixation is an original and ancestral 
feature in both ontogenies, and not as Mortensen 
supposes, ancestral in the Crinoid and secondarily 
intercalated in the Asteroid ontogeny ? 
Of course, the subsequent metamorphosis is very 
different in the two cases—but this difference I have 
correlated with the adoption of different feeding 
habits by two sections of the primordial Echinoderm 
stem. I have the support of Mr. Tate Regan, based 
on his study of a widely different group, that what he 
calls ‘‘ habitudinal differences '’ are the basis of all 
differential evolution. 
With regard to the ‘‘ metagenesis "’ of Echinoderm 
larve, Dr. Mortensen states that in one species of 
Ophiuroid the whole larval body is reproduced by 
the remnant of the ciliated apparatus cast off at 
metamorphosis. This case is certainly unique 
among Echinoderm larve and I cannot accept it 
until Dr. Mortensen brings forward better evidence. 
In any case, it will not, even if true, alter our views 
as to the significance of the larva. May I remind 
Dr. Mortensen that Antedon among Crinoids and 
Amphiura among Ophiuroids can both eject their 
entire alimentary viscera and reduce themselves to 
a framework of arms with a nervous centre and 
yet regenerate all that is lost? Finally, in Dr. 
Mortensen’s appeal to Dr. Bather, he forgets that 
what Dr. Bather objected to was my fathering of 
Dr. Mortensen’s views on him. My friend Dr. Bather 
and I are in substantial agreement in our views on 
Echinoderms. E. W. MacBrive, 
Royal College of Science, South Kensington, 
London, S.W.7, December 18. 


Dr. MortrensEN (NATURE, December 16, p. 806) 
says that ‘'. . . since the larve of the more primi- 
tive Asteroids (the Phanerozonia) are devoid of a 
Brachiolaria stage, the sucking disc . . . must be a 
later specialised structure. ...’’ Surely the state- 
ment is an error, and (even if it were true) the con- 
clusion unjustified. The Phanerozonia of Sladen 
