STAR-FISH AND SEA-URCHINS. 267 
left for its feet to come through, we should have a 
mental picture of an Echinus. It would only be 
necessary to add the curious apparatus of teeth 
(Figs. 40 and 42), which occurs in the Echinus, to 
increase the size of the spines and pedicellarie, and 
to make a few other such minor alterations; but in 
all its main features an Echinus is merely a Star- 
fish with its five rays calcified and soldered together 
so as to constitute a rigid box. 
This echinoid type itself varies considerably 
among its numerous constituent species as to size, 
shape, length and thickness of the spines, ete. ; but 
Fig. 42.—Teeth of Echinus (from Cassell’s * Nat. Hist.’’) 
I need not wait to go into these details. Again, 
merely inviting momentary attention to the develop- 
mental history of these animals, I may remark that 
the phases of development through which an indi- 
vidual 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. 
