118 DR. WILSON ON THE NERVOUS SYSTEM OF THE ASTERID.E 
has on either side a third series (c) with two spines each ; and filling the intermediate space 
between the last rows, two central spines are situated, one behind the other (d). They 
gradually diminish in size from the first series onwards, but are much larger than their 
corresponding neighbours. Those near the eye are tinged with pigment on the ambula- 
cral aspect. When closed, they arch obliquely inwards towards the proximal portion of 
the ray, and cross one another in the same manner as those of the Solaster. In this con- 
dition the tactile organ is found to lie within a circle of spines ; in other words, these 
spines form together a tube or sheath within which the tentacle can be withdrawn during 
contraction, and from which it can be protruded. If these be broken off, the free ex- 
tremities of the lateral calcareous masses, thus exposed, will be found to arch upwards 
and inwards, forming an imperfect ring *. The incompleteness is at the ambulacral aspect; 
and there results from this a deep groove, which contains the lower part or root of the 
tactile organ. It is around this calcareous bridge that the three terminal spine-rows are 
arranged. The groove thus resulting from the incompleteness of the calcareous ring is 
produced by the non- development, centrally, of the coalesced terminal arches. So that 
the bridge is formed, not by the vertebral plates, but by the calcification of the dorsal 
integument and the approximation of the calcareous tubercles found on it. 
Microscopically, the tactile organ presents an external outer coat of circular contrac- 
tile fibres, and an internal longitudinal layer. These muscular layers are attached to the 
free extremities of the calcareous masses (their ambulacral aspect), and probably also to 
the upper part of the terminal arches. They enclose a cavity having more or less the 
form of the organ itself, and lined by a delicate homogeneous membrane f. By careful 
dissection, I have been enabled to trace this cavity beneath the calcareous bridge, and as 
far as the termination of the water-canal (c) X ; whether it possess a vesicle, or not, I have 
not yet decided, but I could not detect any after repeated searching. The best means 
of ascertaining this would be by injecting the water- vascular canal. Numerous nerve- 
filaments pass into its structure from the optic mass of ganglionic cells. 
Whether this be the terminal central tentacle described by Miiller as seen in the deve- 
lopment of some Echinoderms, and also stated by Dr. W. Busch to occur in the young 
Echinaster sepositus §, remains for further investigation to determine. It certainly seems 
to be a modification of the suckers, and, from its great development and the position it 
occupies, may probably constitute potentially a pair or more of these. We have already 
seen that the animal never uses it as a foot. It cannot be an organ for hearing, as the 
cavity in its interior contains no otolithes, but simply a limpid fluid. Its site, on the 
other hand, and the abundant supply of nerves it receives, taken together with the im- 
portant negative conclusions, strongly suggest the hypothesis of its being a tactile organ. 
In the three specimens I have examined, it presents the same general structure, but only 
in the Uraster was I able to study with more certainty its relation to the surrounding 
parts. 
* Plate XV. fig. 8. a. + ibid. fig. 3. b. % Ibid. fig. 8. d. 
% " Beobachtungen uber Anatomie und Entwickelung einiger wirbellosen Seethiere." 1851. My friend Mr. John 
Anderson, who lately directed my attention to the above paper, watched the development of several larvae of Cribella 
oeulata a year ago, and states that this long, contractile terminal tentacle was kept constantly in motion, but never 
nsed as an organ of prehension . 
