104 HERBERT E. DURHAM. 
abundant pigment in cutis and epidermis all round the region 
of the eye, there is none actually over it. 
There is another factor which may be of importance ; this is 
the stimulus afforded by the contents of the cell. Inert 
granules, micro-organisms, pigment concretions, &c., may 
cause the cell to wander further than it ordinarily would. It 
is well known that the coarsely granular leucocytes of Verte- 
brates are the most rapid in their locomotion (vide, e.g., 
Lawdowsky, 44, p. 196, “ Die grobkérnige raschkriechenden 
Leucocyten ”’). 
IV. Notes on EcutnopErRM Histooey. 
So much and such careful work has been done in the past 
few years on the histology of Echinoderms, that it would be 
superfluous to attempt to give anything like an exhaustive 
account of this large subject. I shall confine my attention to 
some points in which my observations do not entirely tally with 
those of other observers. 
This is especially the case with the ‘dorsal organ” of 
Asteroide: no author I have hitherto read has given what 
seems to me to be a satisfactory account of this enigmatical 
organ, and of the tracts of similar tissue which are connected 
with it; the published figures give little or no conception of 
the structure of the organ. My chief investigations have been 
made on Asterias rubens; the other forms mentioned in the 
introduction have also been used: unless otherwise stated the 
descriptions apply to A. rubens. 
As regards preparation, all that need be said here is that the 
dorsal organ is best fixed in situ after exposing it so that 
the preservative agent can act on it well: when fixed it can be 
completely hardened in alcohol or removed from the various 
skeletal portions before that process: decalcification is delete- 
rious to its finer structure. 
Next, it will be advisable to note the nomenclature to be 
used hereafter, on account of the large number of names which 
different authors have used. 
