XV] AND OF SEA-URCHINS 945 



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 multi- 

 tudinous 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 

 equihbrium of forces. But there is this important difference, that 

 an external muscular pressure (such as the .oviduct administers 

 during the consolidation of the 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 probability we require to take into 

 consideration. 



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

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

 in the shell, and, like so many long cables, moor the animal to the 

 ground. They constitute a symmetrical system of forces, with one 

 resultant downwards, in the direction of gravity, and another out- 

 wards in a radial direction ; and if we look upon the shell as originally 

 spherical, both will tend to depress the sphere into a flattened cake. 

 We need not consider the radial component, but may treat the 

 case as that of a spherical shell symmetrically depressed under the 

 influence of gravity. This is precisely the condition which we have 

 to deal with in a drop of liquid lying on a plate ; the form of which 

 is determined by its own uniform surface-tension, plus gravity, 

 acting against the internal hydrostatic pressure. Simple as this 

 system is, the full mathematical investigation of the form of a drop 

 is not easy, and we can scarcely hope that the systematic study 

 of the Echinodermata will ever be conducted by methods based 



