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 (rf). 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- 

 cra! 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 mthin 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 spines form together a tube or sheath within which the tentacle can be withdrawn during 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 dehcate 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) % ; 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 ganghonic 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 constitu.te 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. f Ibid. fig. 3. S. J Ibid. fig. 8. d. 



§ " Beobachtungen iiber 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 

 oculata a year ago, and states that this long, contractile terminal tentacle was kept constantly in motion, but never 

 used as an organ of prehension. 



