516 
PROFESSOR W. THOMSON ON THE EMBRYOGENY OF ANTEDON 
ventral side of the calyx by their base, and may be seen in a constant motion, which 
consists in a sudden inclination upon their base towards the centre, followed immediately 
by a resumption of their more erect attitude.” The interior of the calyx is occupied by 
a reddish-brown visceral mass, obscurely visible through the walls. The author did not 
succeed in getting a view of the mouth, and detected no anal aperture. Dr. Allman 
accurately describes the general structure of the stem ( loc . tit. p. 243) ; he conceives, 
however, that “ the multiplication of the segments of the stem seems to take place 
by the division of the pre-existing ones, and this division seems indicated by the 
transverse ridges, which in several of the segments may be seen running round the 
centre.” 
A detailed description of the developmental stage which forms the subject of Dr. 
Allman’s communication will be found at pp. 525 & 526 of the present memoir. It 
is unfortunate that so able an observer had not an opportunity of making himself 
fully acquainted with this interesting form by the study of a sufficient number of 
specimens. 
In 1856 Professor Sars communicated to the Seventh Meeting of the Scandinavian 
Association a most interesting paper on the Pentacrinoid stage of Antedon Sarsii (Duben 
and Koren). The only specimen observed was dredged on the 14th of March with 
Halicliondria ventilahrum , from a depth of 50 fathoms near Bergen. It was in every 
respect a fully developed Antedon , from the centre of whose centro-dorsal plate proceeded 
a long thin cylindrical articulated stem attached inferiorly to the sponge. The disk with 
its central mouth, the long, cylindrical, excentric anal tube, the radial grooves, the ten 
arms with their characteristic articulations and syzygies, the pinnules with their tentacles, 
the rows of red-brown spots on the margins of the grooves on the arms and pinnules, 
and the dorsal cirri, were completely developed as in the adult form. All the arms were 
unfortunately broken, the portions left bore nine to ten pairs of pinnules. Six of these 
were of the ordinary form ; the three or four proximal pairs, which alternated less regu- 
larly, were setaceous, destitute of tentacles and pigment spots, the innermost pair longer 
than the others, as in the adult ; all the pinnules were attenuated, the generative element 
being as yet undeveloped. The dorsal cirri, twenty to thirty in number, were thickly set 
round the circumference of the centro-dorsal plate. They were fully formed, and the 
joints and terminal claws had the form characteristic of A. Sarsii. The stem was 20 
millimetres in length, and consisted of thirty-one joints ; but as it was broken from its place 
of attachment, some of the inferior joints may have been lost. The two or three lowermost 
joints preserved became shorter towards the base, and the upper joints towards the 
attachment of the stem to the centro-dorsal plate decreased likewise in length ; the 
second joint was about half the length of the third, and the first only half that of the 
second; but the first joint was dilated upwards to its insertion. The middle joints of 
the stem are three to three and a half times longer than wide, and are all dice-box 
shaped like the joints of the dorsal cirri of the species. 
From this observation it would appear that the development of A. Sarsii is continued 
