536 
PROFESSOR W. THOMSON ON THE EMBRYOGENT OE ANTED ON 
part of the subject lightly, as Dr. Carpenter is preparing an elaborate memoir on the 
skeleton of Antedon. I adopt, in concert with Dr. Carpenter, a nomenclature differing 
very slightly from that proposed by M. de Koninck in his valuable work on the fossil 
Crinoids of the Carboniferous System of Belgium. I accept for convenience of descrip- 
tion the division of the body of a Crinoid into three parts, the stem, the head, and the 
arms. The head consists of two hemispheres, a dorsal or apical, and an oral hemisphere. 
The former I shall term the cup of the Crinoid, and the latter the disk. It must be 
remembered, however, that all the radial portions of the head belong morphologically 
and physiologically to the arms. In the earlier stages of development the radial plates 
of the cup, and the radial vessels of the disk, form the budding arms ; and it is only at 
a later period that a distinction is produced between radial and brachial portions, by 
the development of the visceral mass and the extension of the space for its accom- 
modation. 
The mature Antedon has no true stem. The cup is closed beneath by a large circular 
plate hollowed out above into a small rounded chamber. The inferior convex surface 
of this plate in Antedon rosaceus is pitted with a series of small rounded depressions 
perforated in the centre with minute channels communicating with the cavity of the 
plate. Into these depressions are inserted a number of jointed calcareous cirri. I 
shall term the circular plate the “ centro-dorsal plate,” and the appendages the “ dorsal 
cirri.” The centro-dorsal plate in Antedon does not belong to the cup. It represents a 
coalesced series of the nodal stem-joints in the stalked Crinoids. 
In Pentacrinus {Neocrinus) asterias (L.), the stem grows by additions immediately 
beneath the row of basal plates of the cup. These plates are five in number, inter- 
radial, wedge-shaped, their outer wider ends knob-like, heading and corresponding with 
the salient angles of the pentagonal stem. Their inner narrower ends nearly meet in 
the centre, each being only slightly truncated and emarginated, so that the five grooved 
ends may unite in forming the walls of a canal, which is continuous with the central 
canal of the stem, and through which the central sarcode-cylinder of the stem passes to 
branch to special perforations in the first radials. The lower surface of each basal plate 
is hollowed by a longitudinal groove crenated on the edges, and the five grooves are so 
arranged that when the basals are in position, they form together a star-like mould, in 
which the joints of the stem are formed. This cavity holds from three to four stem- 
joints at a time; one extremely small at the bottom of the mould, the others gradually 
increasing in size and gradually forced out and added to the lengthening stem, by the 
growth of those behind them. 
The joints developed in this position are all nodal, that is to say, they subsequently 
bear whorls of cirri. The internodal joints, varying in number in different species, 
are developed afterwards between these, each new internodal joint originating apparently 
immediately beneath the nodal joint. 
The dorsal cirri represent a varying number of compressed whorls of the stem-cirri 
of stalked species which possess such appendages. 
