ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
605 
The author is not able to confirm the general belief that in Holo- 
thurians the plane of symmetry of the young Echinoderm corresponds 
with that of the larva ; they rather cut one another at acute angles. 
The circular and radial canals have taken up their permanent position 
by the eighth day ; the latter arise from the former with a wide lumen, 
and there is no constriction or formation of valves. The distribution 
of the first five tentacular canals is neither radial nor bilaterally 
symmetrical, but asymmetrical in this way that the two tentacles of the 
two ventral interradii receive their water-canals from the median 
ventral radial canal, while the tentacle of the median dorsal, as well as 
that of the left dorsal interradius, is supplied by the left dorsal radial 
canal, while the tentacle of the right dorsal interradius receives its supply 
from the right dorsal radial canal. All the tentacular canals arise from 
the radial canals by a narrow piece which is at first very short, but 
which elongates later ; this opens by a valve into the wider part of the 
canal which lies in the tentacle itself. Notwithstanding its small size 
this valve may be seen to be formed of two semilunar valves, such as are 
already known in the tentacles of Synapta. On either side of the valve 
the enlarged portion of the tentacular canal widens out into a short 
caecum ; this is the foundation of the homologue of the tentacular ampulla 
which Herouard discovered in the adult animal. On the fifteenth day 
the tentacles begin to show signs of their future arborescence. 
The first two feet are laid down on the eighth day ; they are first 
in a pit-like depression of the skin, and have the form of a hemispherical 
projection. On the succeeding days they become more and more tubular, 
and on the fifteenth a well-developed terminal disc becomes apparent. 
Their musculature is a direct continuation of that of the radial canal, 
and is exclusively formed of longitudinal muscular fibres. At the 
origin of the foot-canals there is a valvular arrangement, but it is much 
less well developed than that of the tentacular canals. A third foot 
does not make its appearance till the forty-fifth day, and a fourth not 
until the eighty-fourth. Further increase in the number of feet docs 
not occur till the one hundred and eleventh day. 
Prof. Ludwig does not confirm Selenka’s statement that the Polian 
vesicle lies on the right half of the body, but always on the left, and 
without exception in that left dorsal interradius where Herouard 
constantly found it in the adult. The young stone-canal has a vesicular 
enlargement, which is the commencement of the future madreporic head 
of the permanent stone-canal, and which may, therefore, be called the 
madreporic vesicle ; this is the anterior enterocoel of Bury. On the 
ninety-eighth day of development the vesicle opens by its thin- walled 
side, and so puts the stone-canal into communication with the coelom. 
By the eighth day the central parts of the nervous system are laid 
down ; both the circular nerve and the radials given off from it consist 
at this stage solely of closely packed cells, set in several layers one above 
another. On the next day a finely fibrous layer becomes apparent, the 
fibres of which run parallel with the long axis of the circular nerves. 
I rom the thirteenth day onward separate cells may be found irregularly 
scattered between these fibres. The circular nerve now consists ;of an 
outer cell-layer, and an inner fibrous layer which contains scattered 
cells. As early as the eight day the nervous system ceases to have any 
