[Illustration: Fig. 41.--A portion of the external shell of an Echinus denuded of spines and slightly magnified, showing the arrangement of the plates, the balls in the ball-and-socket joints of the spines, and the holes through which the ambulacral feet are protruded. (From Cassell's "Nat. Hist.")] [Illustration: Fig. 42.--Teeth of Echinus (from Cassell's "Nat. Hist.")] This echinoid type itself varies considerably among its numerous constituent species as to size, shape, length and thickness of the spines, etc.; but I need not wait to go into these details. Again, merely inviting momentary attention to the developmental history of these animals, I may remark that the phases of development through which an individual Echinoderm passes are not less varied and remarkable than are the permanent forms eventually assumed by the sundry species. _Natural Movements._ Turning now to the physiology of the Star-fish group, I shall begin by describing the natural movements of the animals. Taking the common Star-fish as our starting-point, I have already explained the mechanism of its ambulacral system. The animals usually crawl in a determinate direction, and when in the course of their advance the terminal feet of the advancing ray--which are used, not as suckers, but as feelers, protruded forwards--happen to come into contact with a solid body, the Star-fish may either continue its direction of advance unchanged, or may turn towards the body which it has touched. Thus, for instance, while crawling along the floor of a tank, if the terminal feet at the end of a ray happen to touch a perpendicular side of the tank, the animal may either at once proceed to ascend this perpendicular side, or it may continue its progress along the floor, feeling the perpendicular side with the end of its rays perhaps the whole way round the tank, and yet not choosing, as it were, to ascend. In the cases where it does ascend and reaches the surface of the water, a Star-fish very often performs a number of peculiar movements, which we may call acrobatic (Fig. 43). On reaching the surface, the animal does not wish to leave its native element--in fact, cannot do so, because its sucking feet can only act under water--and neither does it wish again to descend into the levels from which it has just ascended. It, therefore, begins to feel about for rocks or sea-weeds at the surface, by crawling along the side of the tank, and every now and then throwing back its uppermost ray or rays along the surface of the water to feel for any solid support that may be within reach. If it finds one, it may very likely attach its uppermost rays to it, and then, letting go its other attachments, swing from the one support to the other. The activity and co-ordination manifested in these acrobatic movements are surprising, and give to the animal an almost intelligent appearance. [Illustration: Fig. 43.--Natural movements of a Star-fish on reaching the surface of water.]