, we ee ee 
| Marcu 10, 1923] 
NATURE 
323 

_ numerous Astropectinids live exclusively on a muddy 
- bottom, and also that numerous Spinulosa and 
_ Forcipulata live on a sandy or muddy bottom. 
Prof. MacBride states that my appeal to Dr. 
Bather’s ig 8 is quite mistaken, because I forget 
_ that “what Dr. Bather objected to was my [Prof. 
_ MacBride’s) fathering of Dr. Mortensen’s views on 
_ him.” May I only quote the following sentence from 
_ Dr. Bather’s reply: “‘ It is not for me to break any 
lances fin defence of Dr. Mortensen, but if Prof. 
_ MacBride is acquainted with Dr. Mortensen's ‘ Studies 
_ in the Development of Crinoids’ ... I am rather 
astonished that he should so belittle our Danish col- 
_ league’s workon those lines.’’ If thissentence is meant 
_ by Dr. Bather to express his substantial agreement 
_ with Prof. MacBride in their views on Echinoderms or 
_ to repudiate Prof. MacBride’s fathering of my views 
_ on him, I am very sorry that I shall have to moderate 
very considerably the admiration which I have always 
had for his lucid way of expressing his opinions. 
To Prof. Gemmill’s remarks (NATURE, January 13, 
p. 47) I must reply very decidedly that I am not 
narrowing down the Phanerozonia to include only 
the family Astropectinide. The families Luidiide, 
Archasteridz, and Goniasteride, at least, are likewise 
_ typical Phanerozonia. On the other hand, the position 
_ of the Asterinide and the Gymnasterid@ is just one of 
_ the weak points in the classification of Asteroids, and 
the latter can by no means be said to be “ frankly 
_ Phanerozonate.”’ 
The conclusion that, since the larve of the two 
families, Astropectinide and Luidiide (not of the 
Astropectinide alone as Prof. Gemmill states, by inad- 
_ vertence, of course), regarded (by most specialists on 
_ Asteroids) as the more primitive forms, have no 
sucking disc, the existence of such a disc in the larve 
of those groups regarded (by most specialists on 
Asteroids) as more specialised types, is a secondary 
: adaption, may, possibly, not be “‘ inevitable ’’ ; but, 
_ in any case, this conclusion is not illogical or absurd. 
I have no direct interest in maintaining the Brachio- 
laria to be a secondarily specialised larval type. If 
conclusive proof is given that the Brachiolaria is the 
primitive, the Astropectinid-larva the specialised 
‘orm, I shall not hesitate to drop my present view. 
- But [ must maintain that this view is not unjustified 
by the facts so far known. 
I am sure Prof. Gemmill will agree with me as to 
_the desirability of researches on the development and 
metamorphosis (and, not least, the postembryonal 
development) of many more forms than those few, 
which have been studied up to now. Not even the 
development and metamorphosis of Asteropecten has 
been studied by means of modern methods, the re- 
searches of Joh. Miller and Metchnikoff still remain- 
ing the only base of our knowledge of this subject. 
I hope very sincerely that Prof. Gemmill will extend 
his admirable studies to this and many other 
Asteroids, as I also hope that both he and Prof. 
MacBride will agree that my efforts to widen our 
knowledge of the development of Echinoderms are 
not entirely without value, and that the views 
expressed in my work, however much they may dis- 
agree in them, are not entirely without reasonable 
foundation. : TH. MORTENSEN. 
Zoological Museum, Copenhagen, 
January 22. 







I SHALL summarise the points at issue between 
Dr. Mortensen and myself as briefly as possible. 
__ Hecomplains that I made an “‘ unprovoked personal 
_attack”’ on him. Nothing was further from my in- 
tentions. The so-called attack was a criticism of 
NO. 2784, VOL. 111] 
habits. 
certain views attributed to Dr. Mortensen by Dr. 
Bather in a review of one of Dr. Mortensen’s recent 
works in Nature. Dr. Bather seemed to think that 
Dr. Mortensen believed that after all the development 
of Echinoderms might be an alternation of generations 
as Johannes Miiller originally suggested. As Dr. 
Mortensen has unreservedly repudiated this view 
‘there is nothing more to be said on this point. 
But Dr. Mortensen did say that the fixed stage in 
the development of Asteroids (discovered by me in 
1893) was of secondary character, because it was 
absent in two families (Astropectinide and Luidiidz) 
classed together as Paxillosa. I had a perfect right 
.to comment severely on statements such as these, 
because (1) the fixed stage is found in the most widely 
diverse families belonging to two of the great primary 
divisions of Asteroidea. (2) The fixed stage regarded 
as an ancestral reminiscence, enables us to understand 
how and why the ancestors of Asteroidea passed from 
the stage of free-swimming bilaterally symmetrical 
animals to the stage of radially symmetrical forms 
creeping over the bottom. (3) If Dr. Mortensen had 
known what he as a specialist in Echinoderms might 
reasonably be expected to know, namely, what has been 
determined as to the physiology and habits of Luidia 
and Astropecten, he could never have regarded them 
as primitive, but would have recognised them as what 
they are, the most specialised of all Asteroidea. 
It is not a question of the ground on which parti- 
cular starfish can be dredged up. Every marine 
biologist knows that sporadic individuals of rock and 
gravel-inhabiting species can be dredged on sand or 
mud. The dredge, indeed, gives no precise information 
as to the habitat of a species, for the bottom is usually 
“patchy.” But Luidia and Astropecten when observed 
in life are found to be burrowing species, which when at 
rest are almost completely buried in the sand or mud 
in which they live like many Ophiuroids, and the 
structure of the arms is modified in relation to such 
A fixed stage in the ontogeny of such forms 
would be an impossibility, for in such an environment 
the larva would find nothing to which it could attach 
itself. By a happy coincidence I received a few 
days ago Part V. of W. K. Spencer’s “ Paleozoic 
Asterozoa.’’ In this work I read ‘‘ The existence of 
large marginals throws no light on the affinity of 
extinct species, but it does throw light on the shape 
of the arm”’ (i.c. it is adaptive). When the arm is 
flat and the dorsal skeleton reduced to a flexible mem- 
brane the borders of the arms must be strengthened. 
Dr. Mortensen accused me of referring contemptu- 
ously to certain specialist students of external features 
only. I am afraid I must plead guilty on this count. 
I have spent weary time in going through the ponder- 
ous works of Sladen and Ludwig, and so far as any 
‘attempt to correlate structure with function is con- 
cerned, these authors might just as well have been 
describing postage stamps as Asteroidea. The 
distinction between Phanerozonia and Cryptozonia 
was made by Sladen. The Phanerozonia were stated 
by him to be the original and primitive group (a) 
because fossil starfish were all phanerozonate, (b) 
because cryptozonate forms when young are phanero- 
zonate. 
I have never been able to find the evidence on 
which (b) is based. I have often seen young imagines 
of Asterias and Asterina, but there is certainly nothing 
“‘phanerozonate’’ in their appearance. Statement 
(a) is absolutely inaccurate. If Dr. Mortensen is open 
to conviction on this point let him study W. K. 
Spencer’s monograph, where he will find every fossil 
form from Paleozoic strata carefully described, and 
further, an attempt made to correlate its structure 
with its probable habits. He will learn that Crypto- 
zonia are just as old as Phanerozonia, and that the 
