404 
ORIGINAL ARTICLES. 
form which he obtained at Helsingfors, in 1847. Brachiolaria is in 
many respects closely allied to Bipinnaria, but it presents certain 
structural peculiarities which I regard as of the greatest importance, 
bringing the Bipinnarian form into relation with some apparently 
extremely dissimilar forms of Echinoderm development to be described 
hereafter. 
The individuals observed were about 3/4ths of a line in length. 
The general arrangement of the appendages, of the ciliated fringes, 
and of the mouth and alimentary canal were essentially the same as 
in Bipinnaria, but the anterior extremity instead of ending in two 
lobed fin-like organs, was produced into three tubular appendages, 
each ending in a minute sucker crowned with small papillae. One 
of these tubular feet was nearly terminal, rather towards the dorsal 
aspect of the animal, the other two were turned downwards. The 
walls of these tubes were highly contractile, the animal using them 
freely as accessory locomotive organs. The walls were ciliated in¬ 
ternally, and the cavity contained a corpusculated fluid. Muller ob¬ 
serves, that the cavity of the tubular appendages was continuous 
with the cavity of the body. There was a dark oval granular patch 
in the centre between the three arms. 
The posterior extremity of the pseudembryo which contained the 
stomach was much thickened, and formed a five-lobed granular disk 
placed somewhat obliquely to the axis of the pseudembryo. The 
dorsal surface of this disk, the surface turned from the centre of the 
pseudembryo, was arched and gibbous, and was supported by a net¬ 
work of calcified areolar tissue. The ventral surface was flattened, 
and within it a five-rayed star of vascular coeca might be detected, 
the rudiments of the ambulacral ring of the starfish. 
20. The development of this form was not traced further. Prof. 
Muller at first believed tha/t the posterior granular disk was the com¬ 
mencement of the perisom of the starfish, but he afterwards came 
to the conclusion from the analogy of Bipinnaria, that the granular 
disk and the calcarous network were special to the pseudembryo, and 
part of its structure, and that the Echinoderm must be expected to 
arise as a separate bud. I have myself no doubt whatever, after 
studying the development of Asteracanthion (§ 27) that Muller’s first 
view was the correct one. I believe that in this case, only part of 
the germ-mass was converted into the pseudembryonic sarcode, and 
that simultaneously with the development of the pseudembryonic 
appendages, another portion, the granular disk, was being modified 
into the perisom and tissues of the starfish. 
The three hollow arms present all the characters of the ambula¬ 
cral feet of the starfish, and I think there can be no doubt, that they 
are in connection with a cavity in the body of the pseudembryo ho¬ 
mologous with the ciliated coeca of Bipinnaria , and that they are 
consequently true appendages of the nascent ambulacral system of 
the starfish. These temporary vascular appendages for locomotion 
and aeration, naturally assume the form afterwards repeated in the 
ambulacral water-feet. 
