PROFESSOR WYYILLE THOMSON ON THE ECHINODERMATA. 405 
21. The body in BrocJiiolaria is thus apparently divided into 
three portions. The posterior portion includes the common stomach, 
and consists chiefly of a part of the original germ-mass becoming de¬ 
veloped into the Echinoderm embryo. The middle portion contains 
the mouth and oesophagus, and a ciliated cavity homologous with 
the special blood-cavity in Bipinnaria, and has its sarcode substance 
extended into the usual appendages bordered by the usual ciliated 
fringes. The anterior portion is an extension of the ciliated sac 
into three temporary and locomotive water-feet. The peripheral 
sarcode layer is continuous over all. 
It is unfortunate that it has been hitherto found impossible to de¬ 
termine to what species of starfish, either the various forms of Bi¬ 
pinnaria, or the Brachiolaria belong. 
III. — Asteracanthion violaceus, Asteracanthion Mulleri, 
AND EcHINASTER SANGUINOLENTUS. 
22. In this group of star-fishes no free swimming pseudembryo 
is produced. The eggs are developed in a kind of “ marsupium,” 
formed by the curving inwards over the mouth, of the rays of the 
parent. The eggs pass into this cavity, and before they are fin ally 
extruded the young starfishes are fully formed. 
23. I observed with care, during the winter of 1860, # the de¬ 
velopment of the young of Asteracanthion violaceus (M. & T.). 
In this species segmentation of the yelk is complete. After segmen¬ 
tation the germ-mass is at first spherical, finely granular, and still 
invested by the vitelline membrane. The membrane soon disappears, 
and within a few hours the embryo seems perfectly homogeneous, 
regularly oval, and of a delicate flesh colour. I could not detect the 
slightest trace of cilia on the surface. Eour or five hours later, the 
oval form is still more marked ; one end has become slightly dilated, 
and towards this end there is an accumulation of the denser part of 
the granular substance. The whole embryo is now invested by a 
delicate, structureless, gelatinous layer, which is thinner and less 
apparent toward the narrower and more transparent end of the oval. 
At the broader end it invests a dark, consistent, granular layer of 
considerable thickness, formed of oil-globules and compound granular 
masses and cells, which lines a central cavity filled with a clearer 
granular semi-liquid, in which there are traces of molecular or ciliary 
motion. 
The embryo now becomes club-shaped, and there is a decided 
aggregation of the great mass of the granular matter to the thick 
end of the club, whose transparent investing membrane becomes still 
more distinct, and the internal granular layer thicker. 
24. The transparent investment of the narrow end protrudes one 
and then two more tubercles, which rapidly declare themselves 
* “ On the Embryology of Asteracanthion violaceus (M. & T.)”—Q. J. M. Soc. 
1861. 
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