746 
SUMMARY OP CURRENT RESEARCHES RELATING TO 
Echinodermata. 
Morphology of Echinoderms.* — M. L. Cuenot finds that in the 
course of development of Ophiuroids the ectoderm of the walls of the 
body is intermingled with the mesenchyme in such a way that distinction 
between them is impossible : in the adult the primitive ectoderm can 
only be recognized at certain points, such as the tentacles and teeth. In 
Cucumaria , likewise, the wall of the body is not bounded by ectoderm, 
for that layer is imbedded in the subjacent mesenchyme, where it forms 
groups of cells, above which are the connective fibres that form the 
outer covering of the body. In Elpidia glacialis there is no distinct 
ectoderm at all. In all Echinoderms the calcareous matter is formed in 
the same way ; it is deposited on a connective plexus, in which nuclei are 
scattered, and which may be seen after decalcification ; it is secreted 
by mesenchymatous cells which are very abundant in all developing 
calcareous tissues ; the holes are due to the plexiform arrangement, and 
not, as Herouard thinks, to the presence of nuclei. 
The small ciliated spines which invest the fascioles of Spatangoids 
are identical with the vibratile spinelets which the author has described 
in Astropecten ; like them, they appear to facilitate the renewal of water 
either around the anus (circumanal fasciole) or the branchiae (circum- 
petalous fasciole). The anchors of Synaptids have no muscular fibres 
and play a passive part in locomotion, like the hooks of Ophiurids. 
Such Clypeasterids as were examined were found to be all provided with 
small tridactyle pedicell arise, which recall those of Spatangoids. The 
Cuvierian tubes of Holothurians cannot be considered as anything else 
than defensive organs, and can be expelled in large numbers without 
injuring the digestive tube. 
The author has studied the invagination of the nerve-cords in 
AmpJiiura squamata ; at first they are superficial and exactly resemble 
those of Asterids, but by a process which is more like epiboly than 
invagination, tegumentary folds are formed above the nerve-ring, the 
radial cords, and the basal part of the ambulacra, and inclose a smaller 
portion of the external medium which forms the system of epineural 
cavities. It is possible that in some palaeozoic Ophiuroids the nervous 
system was superficial. In all Echinoderms the histological constitution 
of the central parts of the nervous system is the same — nerve-fibrils 
running between the base of long filiform cells, the nucleus of which is 
placed near the exterior. These ectodermal cells, notwithstanding their 
epithelial form, appear to play the part of ganglionic nerve-cells. 
There are a number of vestiges of the nervous invagination ; there 
may be an empty space above the oral ring and the radial cords, or there 
may be, in youth, a direct continuation between the oesophageal epithe- 
lium and the oral ring already inclosed in the tissues ; as the animal 
grows older the communication becomes reduced, and may altogether 
disappear. Thirdly, the radial cords always fuse with the ectoderm at 
their extremities. 
The central nervous system consists of an outer and more important 
part, formed by the ectoderm, and an inner part, less constant, much 
* Arcli. Zool. Exper. et Gen., ix. (1891) pp. viii.-xvi. See also Arch, de Biol., 
xi. (1891) pp. 313-504 (4 pis.). 
