EMBRYOLOGY AND ASEXUAL REPRODUCTION. Edl. 37 
ventral interradii arise from the median ventral radial vessel ; the ten- 
tacles of the median dorsal and left dorsal interradii arise from the left 
dorsal vessel ; and the tentacle of right dorsal interradius from the right 
dorsal vessel. Hence the five primary tentacles of Holothurians cannot, as 
Semon supposed, determine the true radii of the Holothurian body. Two 
more tentacles arise, at a much later period, from the right and left 
ventral vessels, and lie in the left and right dorsal interradii. The 
tentacles are arranged just as in the seven-tentacled young of Chirhlota 
rotifcra. In the adult Cixcumaria each radial vessel gives off two tenta- 
cular canals. The valves in the tentacles consist of two semilunar folds. 
Above the valve a ccecum arises, which becomes the ampulla. Longi- 
tudinal muscle-fibres appear in the upper expanded portion of the 
tentacles, formed by the cells of the hydroccele. The tentacles are 
simple cylindrical structures, with rounded tips, up to the fifteenth day, 
on which they begin to branch. Rudiments of the first two tube feet are 
present on tho eighth day, the right foot being slightly in front of tho 
left. A third foot appears much later, and a fourth still later, both 
from the median ventral vessel. The fifth foot arises from left dorsal 
vessel on its loft (ventral) side. The Polian vesicle lies in tho left half 
of the body, and its hydrocoelic epithelium forms circular musclc-s. The 
young stone canal has a vesicle-like expansion, consisting of flattened 
epithelium in its outer half. This expansion— the anterior enterocoele 
of Bury — is the first rudiment of the subsequent madreporic head, and 
may be termed “ madreporic vesicle.” It is surrounded with a lattice-work 
envelope, formed by the mesenchyma. The distal end of the stone 
canal and the water-pore lie to the right of the dorsal mesentery. On the 
ninety-eighth day the madreporic vesicle acquires an opening into the 
body cavity, and the distal end of the stone canal and water-pore atrophy. 
Rudiments of the nerve-ring and the radial nerves exist on the eighth 
day. When fully developed, the nerve-ring and radial nerves consist of 
a superficial layer of cells, and beneath this a layer of fibres sheltering 
scattered cells. On the ninth day the tentacular nerves arise, interradial 
in origin. As early as the eighth day the nervous system of the young 
animal has lost its connection with the ectoderm, from which it is sepa- 
rated by an intervening layer of mesenchyme. The outer surface of the 
nerve-ring and radial nerves does not come into contact with the mesen- 
chyme, but is separated by a cleft, which persists as the epineural ring 
and canals, which are in free communication with one another, but not 
with any other cavity of tho body. Epineural spaces also accompany 
the tentacular and podal nerves. The radial nerves are at first in contact 
with the radial water vessels, but later a fine cleft appears between them 
which is probably the rudiment of the “ pseud-hcemal canal.” Nothing 
could be observed of tho perpendicular fibres, transverse septum, or the 
two cellular columns of the nerve-cord, which are hence probably 
secondary acquisitions. No auditory organs were found. The body 
musculature is formed from the cells of the parietal enterocoele : first the 
ventral longitudinal muscle, then the transverse muscular layer, and after- 
