44 Prof. Duncan on the Nervous System of Actinia. [Dec. 18, 



giving each spot a weight in proportion to its area, the variation of 

 these positions in latitude and longitude, and their weights, might give 

 a more satisfactory base for this comparison and for other deductions. 



It will be obvious also that this investigation refers only to the visible 

 hemisphere of the sun ; an approximation to the spot-distribution, on 

 the other hemisphere, however, will be frequently possible. 



II. " On the Nervous System of Actinia." Part I. By Professor 

 P. MARTIN DUNCAN, M.B. Lond., F.R.S., &c. Received 

 October 9, 1873. 



(Abstract.) 



After noticing the investigations of previous anatomists in the his- 

 tology of the chromatophores, the work of Schneider and Rotteken on 

 these supposed organs of special sense is examined and criticised. 



Agreeing with Rotteken in his description, some further information 

 is given respecting the nature of the bacillary layer and the minute 

 anatomy of the elongated cells called "cones" by that author. The 

 position and nature of the pigment-cells is pointed out, and also the pecu- 

 liarities of the tissues they environ. It is shown that the large re- 

 fractile cells, which, according to Rotteken, are situated between the 

 bacilli and the cones, are not invariably in that position, but that bacilli, 

 cones, and cells are often found separate. They are parts of the ecto- 

 thelium, and when conjoined enable light to affect the nervous system more 

 readily than when they are separate. Further information is given 

 respecting the fusiform nerve-cells and small fibres noticed by Rb'tteken 

 in the tissue beneath the cones ; and the discovery of united ganglion-like 

 cells and a diffused plexiform arrangement of nerve is asserted. The 

 probability of a continuous plexus round the Actinia and beneath each 

 chromatophore is suggested, and the physiological action of the struc- 

 tures in relation to light is explained. 



The minute structure of the muscular fibres and their attached fibrous 

 tissue in the base of Actinia is noticed ; and the nervous system in that 

 region is asserted to consist of a plexus beneath the endothelium, in 

 which are fusiform cells and fibres like sympathetic nerve-fibrils. More- 

 over, between the muscular layers there is a continuation of this plexus, 

 whose ultimate fibrils pass obliquely over the muscular fibres, and either 

 dip between or are lost on them. 



The other parts of the Actinia are under the examination of the 

 author, but their details are not sufficiently advanced for publication. 

 The nervous system, so far as it is examined, consists of isolated fusi- 

 form cells with small ends (Rotteken), and of fusiform and spherical 

 cells which communicate with each other and with a diffused plexus. 

 The plexus at the base is areolar ; and its ultimate fibres are swollen her 

 and there, the whole being of a pale grey colour. 



