4 ECHINODEKMA. 



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, 

 Ophiothriv 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 precis 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. Gr. 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." 



