PEOEESSOE WTVILLE THOMSON ON THE ECHINODEEMATA. 403 
distinct bilateral symmetry ; and, but for the presence of tlie ciliated 
sacs it might be conceived to homologate with some low or larval 
forms of the Annulosa. It is developed from the egg of a starfish 
under the conditions usual to the development of the embryo, but it 
is not the embryo of the starfish. Its life is quite distinct from that 
of the starfish, and it never becomes converted into it by any pro¬ 
cess of ecdysis or metamorphosis. 
It is simply an embryonic appendage* * intended like the embryonic 
appendages of the higher animals, to absorb and elaborate nourish¬ 
ment for the nascent embryo, and it is enabled to assume its inde¬ 
pendent zooidal form by virtue of the singular properties of the 
simple histological element of which it is composed. 
18. I cannot regard the Echinoderm embryo as in any ordinary 
sense a bud from the pseudembryo. The tissues of the young star¬ 
fish seem to me to be in all cases formed and arranged according to 
their special developmental law, within the sarcode pseudembryonic 
substance. The dorsal vessel with its rosette of coeca which appears 
so early in the development of some bipinnarian forms cannot, I 
think, be considered as an inversion of the dorsal integument. The 
pseudembryo possesses no true integument. The external surface is 
merely bounded by a firmer layer of the structureless gelatinous sub¬ 
stance, and the primordial vessels are hollowed out just as the 
ambulacral ring is hollowed out in the sarcode substance of species 
which progress to their definite Echinoderm form without the develop¬ 
ment of any pseudembryonic appendages.* In the starfish of 
Bipinnaria asterigera the sarcode layer becomes much attenuated 
over the surface of the disk, but in the Erieste (§ 12.) Bipinnaria the 
relations of the embryo as a true internal development are sufficiently 
apparent. In Comatula ,f this relation is still more evident. The 
embryo is developed entirely within the body of the pseudembryo 
without any connection whatever with the perisom of the latter. 
II.— The Beachiolakia. 
19. Professor Muller describes J under this name, a pseudembryonic 
* Professor Carpenter was, I believe, the first to propose this view in a definite 
form, and I consider it so important as affording a key to the true nature of this 
singular series of phenomena, that I transcribe his words in full. “We here find the 
“ yolk-mass converted into a structure, which is destined only to possess a transient 
“ existence, and which disappears entirely by the time that the development of the 
“ off-set from it has advanced so far, that it begins to assume the characters of the 
“ permanent organism. This, however, is what takes place in the higher vertebrata; 
“ for the structures first developed in the egg of the bird hold nearly the same rela- 
“ tion to the rudimentary chick, that the ‘ Pluteus ’ bears to the incipient Echinus 
“ or Ophiura, or the Bipinnaria to the incipient Starfish. 5 ’— Principles of Com¬ 
parative Physiology. P. 568. 
* Wyville Thomson—“ On the development of Synapta inhaerens (O. F. 
Muller.)”—Q. J. Mic. Sc. 1861. * 
f Wyville Thomson. “ On the Embryology of Comatula rosacea (Linck.)” 
Proc. Royal Society, 1863. 
J “ Ueber die Larven und die Metamorphose der Echinodermen.”—Berlin, 1849. 
N. H. R.—1863. 2 E 
