CEPHAI^OPODA OF THE HAWAIIAN ISLANDS. 
269 
or apology even to the layman. This is particularly needful concerning the Octopoda, 
as in this group it is often very difficult to lay down on paper a hard and fast line respect- 
ing a given character which will invariably serve to separate a species from some other 
perhaps very closely allied. Here, in the absence of direct comparison between speci- 
mens, a sort of average of the entire physiognomy is largely depended upon, taking into 
consideration the relative length of the arms, the shape and extent of the umbrella, 
the presence or absence of ornamental processes of various sorts upon the integument, 
and the more evident conditions of preservation, which frequently affect the features 
already stated to a degree which can scarcely be too strongly emphasized. When the 
specimens are males, however, the structure of the hectocotylized arm is variously 
modified, usually constant in its peculiarities, and hence a criterion of the highest import- 
ance; perhaps no other single feature so well maintains its value in preserved material. 
I am also inclined to treat with respect any decided peculiarities of color, particularly 
when the various pigments appear to be disposed after the manner of a definite pattern. 
Frequently the so-called “funnel organ” exhibits tangible modifications in shape, though 
whether these latter are more valuable than confusing as a key to interrelationship is not 
yet apparent. 
Among the Decapoda some of the above-mentioned features are of very minor 
specific significance. Here I have frequently given considerable weight to relatively 
minute differences in the form and arrangement of the suckers (or hooks) on the sessile 
arms and more particularly, on the terminal clubs of the tentacles, as these characters, 
even though small, are definitely to be apprehended and usually little affected by the 
action of the preserving medium. Nevertheless, there is certainly need for a greatei 
quantity of comparative data showing the range of variation in these organs and until 
this is available there is constant danger that too much emphasis has been placed upon 
their details. Among the luminous forms, particularly the CEgopsida, the arrangement 
and structure of the photogenic organs is nearly always subject to important modification, 
both specific and generic or even of higher significance. The funnel organ appears to 
be of less practical systematic value in the Decapoda than in the Octopoda, for although 
it undergoes considerable modification among different genera (particularly the various 
Cranchiidae), the differences between those of closely allied species are apt to be too 
slight to be appreciable. 
On account of their easy preservation as “hard parts,” the gladius and radula have 
occupied a dominant position in the schemes of classification of many of the older authors 
as well as a few of the more recent ones, but the undesirable mutilation of the specimen 
entailed by their examination, coupled with the writer’s belief that their relative import- 
ance has been greatly overrated, has in the present work prevented their receiving the 
attention to which they are perhaps more justly entitled. Other “hard parts” which 
are ofttimes very useful to observe are the horny rings which arm the apertures of the 
suckers in most if not all Decapoda and which often show interesting modifications, 
chiefly depending upon the var3dng degrees of smoothness or denticulation of their outer 
margins. 
60289°^ — Bull. 32 — 14 — — 18 
