ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 849 



sensory cells are connected together with the nerve- fibrils and 

 ganglionic ceils into a whole, which forms the nervous system. The 

 nervous layer forms a circle around the mouth and sends out a mass 

 into each arm or ray. This arrangement cannot but call to mind 

 that which obtains in the Coelenterata. 



Interesting as the resemblance is, it becomes more so when we 

 reflect on the want of genot'c relationship between these two phyla ; 

 in the two cases the nervous system must have been independently 

 developed ; and we gain new ideas in confirmation of certain doctrines 

 as to the genesis of nerves and nervous systems. The brothers 

 Hertwig have taught us that the most primitive arrangement is that 

 in which cells, sensory, muscular, and ganglionic cells, are also 

 epithelial. By their position these cells are able to react directly to 

 external stimuli ; the epithelio-muscular cells have given off con- 

 tractile fibrils, the epithelio-ganglionic cells have numerous connections 

 with one another, and with the sensitive and muscular cells, while the 

 sensory cells are specially adapted for receiving sense-impressions. 

 Dr. Hamann thinks that this is just the very condition which is to be 

 observed in the case of the Asteroidea. The view that the specific 

 sensory organs have arisen from indifferent sensory cells was sug- 

 gested to the Hertwigs by their study of the Medusae ; an examination 

 of the optic organs of star-fishes leads to a similar conclusion, for in 

 them the eye-spot consists of groups of collected sensory cells, in 

 which a pigment is deposited ; this is the simplest condition of an 

 optic organ in the whole of the animal kingdom, not excepting even 

 the Medusae. When we reflect that sensory cells — the necessary 

 foundation for the genesis of specific sensory organs — are scattered 

 over the whole surface of the body, we shall not be astonished at the 

 phenomena of sensory organs appearing in the most various regions 

 of the body. 



Acanthology of Desmostichous Echinids.* — H. W. Mackintosh, 

 in his second report on this subject, commences with an account of 

 the spines of the family Arbaciidce ; in Arhac'm the results of his studies 

 would lead him to take a different view of the relationships of the 

 species to that proposed by A. Agassiz. Too few representatives of 

 the Echinometridse have been studied for the author to be able to 

 regard his work as a satisfactory investigation of the group ; and the 

 same is true of the Echinidse ; some information is, however, afforded, 

 and drawings are given of sections of spines of species of which, as 

 yet, no account has been published. From what Mr. Mackintosh 

 says it appears that many of the figures given in Prof. Agassiz's 

 ' Eevision ' are very unsatisfactory, and it is clear that the whole 

 subject requires to be carefully and continuously worked out, and 

 that there is here an opening for an observer who wishes to devote 

 himself to microscopical investigation. 



In his third report "j" some corrections of earlier statements and 

 some brief notes on newly discovered Diadematidse and Echinothuridse 

 are to be found. 



* Trans. K. Irish Acad., xxviii. (1883) pp. 241-58 (4 pis.). 



t Ibid., pp. 259-66 (2 pis.). These two reports are issued separately. 



