662 ON THE SHAPES OF EGGS [ch. 



slight details which do not aft'ect our problem, have a complete 

 radial symmetry. The axis of the animal's body is vertical, 

 with mouth below and the intestinal outlet above; and around 

 this axis the shell is built as a symmetrical system. It follows 

 that in horizontal section the shell is everywhere circular, and we 

 shall have only to consider its form as seen in vertical section or 

 projection. The irregular urchins (very inaccurately so-called) 

 have the anal extremity of the body removed from its central, 

 dorsal situation ; and it follows that they have now a single plane 

 of symmetry, about which the organism, shell and all, is bilaterally 

 symmetrical. We need not concern ourselves in detail with the 

 shapes of their shells, which may be very simply interpreted, by 

 the help of radial co-ordinates, as deformations of the circular or 

 "regular" type. 



The sea-urchin shell consists of a membrane, stiffened into 

 rigidity by calcareous deposits, which constitute a beautiful 

 skeleton of separate, neatly fitting "ossicles." The rigidity of 

 the shell is more apparent than real, for the entire structure is, 

 in a sluggish way, plastic; inasmuch as each httle ossicle is 

 capable of growth, and the entire shell grows by increments to 

 each and all of these multitudinous elements, whose individual 

 growth involves a certain amount of freedom to move relatively 

 to one another ; in a few cases the ossicles are so little developed 

 that the whole shell appears soft and flexible. The viscera of the 

 animal occupy but a small part of the space within the shell, the 

 cavity being mainly filled by a large quantity of watery fluid, 

 whose density must be very near to that of the external sea-water. 



Apart from the fact that the sea-urchin continues to grow, it 

 is plain that we have here the same general conditions as in the 

 egg-shell, and that the form of the sea-urchin is subject to a similar 

 equilibrium of forces. * But there is this important difference, that 

 an external muscular pressure (such as the oviduct administers 

 during the consoUdation of egg-shell), is now lacking. In its 

 place we have the steady continuous influence of gravity, and 

 there is yet another force which in all probabihty we require to 

 take into consideration. 



While the sea-urchin is alive, an immense number of delicate 

 " tube-feet," mth suckers at their tips, pass through minute pores 



