4 ECHINODERMA. 
enough in shallow pools, and may sometimes be seen living for 
a time completely out of the water, on ridges and ledges of rock. 
But water is their element, and they can neither breathe nor 
feed unless they are in it. They move slowly and often remain 
long in the same position; they are eminently gregarious, and 
it is quite a common occurrence for the dredger to come upon 
immense numbers of one species. This gregarious nature is of 
enormous assistance to the student who is interested in the 
variation of species, a study far more interesting and fertile 
than that of new forms, which was almost the sole object of the 
older school of naturalists. The most casual observer cannot 
fail to be struck by the wonderful differences in hue and tone 
exhibited by some, and particularly by the common Brittle-star, 
Ophiothrix pentaphyllum ; a little closer examination will show 
that in the arrangement and numbers of rows of spines, in the pores 
that perforate some of the plates of the skeleton of an Urchin, and 
so on, there is a similarly wide range of variation. The difficulty 
of drawing up diagnoses of Echinoderms comes, therefore, to be 
considerable, and it must not be imagined by the collector that 
all those specimens which do not rigidly conform to the diagnoses 
that follow are examples of new species. The student can make 
no greater mistake than attempt to draw up long descriptions, which 
come, of course, to be descriptions of individuals, until he has first 
concisely and intelligibly presented a diagnosis or précis of the 
specific characters of the form before him *. 
Some Echinoderms prefer rocky, others sandy shores ; in muddy 
places they are often rare, if not absent. They are voracious, and 
though carnivorous, and even cannibal, they do not disdain vege- 
table food. The Holothurians and the Crinoids are unable to tear 
up their food: the former have no means of comminuting it, and 
can only obtain comparatively small objects by means of the tentacles 
by which the food is brought to the mouth; in Crinoids the food, 
which is much more delicate and minute, consists only of micro- 
scopic organisms which are swept down to the mouth along the 
grooves on the lower or oral side of the arms; the movement is due 
to the cilia which are found in abundance in these grooves. 
Digestive Organs.—The Starfish and the Sea-Urchin are exceed- 
ingly voracious, and shell-fish and crustaceans of some size easily 
fall victims to their rapacity: the former is provided with a large 
stomachal sac capable of considerable protrusion and able to en- 
velope a shell, and to draw it and its inmate within the area of 
the disk; or, dividing its arms into two sets, it will pull aside the 
valves of an oyster and drag from within the maker of the shell. 
* Compare the words of Sir J.G. Dalyell (‘ Powers of the Creator,’ i. p. 92):— 
“The aspect of individuals often alters very much, either superficially, or in 
the distribution of the colours. Some undergo a great external change with 
age. The whole tribe seems to abound in varieties, insomuch that it is difficult 
to reconcile the observations and descriptions of different naturalists. Perhaps 
the enumeration of species exceeds the truth of Nature.” 
