By the Rev. T. A. Preston. 91 



plates, the central aperture being occupied with the extremities of 

 five teeth, which project slightly beyond the general surface of the 

 animal, and around the mouth — attached to the membrane (called 

 the buccal membrane) — are five small branched gills. 



To each of the bosses scattered over the inter-ambulacral area is 

 attached a spine, very variable in shape and size, not only in different 

 species, but also in the same animal. Sometimes the spines are 

 very thin and slender, presenting more the appearance of hairs, at 

 others they are large and stout, as much as three inches long. These 

 spines have a very beautiful structure, and sections of them make 

 very interesting microscopic objects. They serve partly for pro- 

 gression and partly for enabling the echinus to bury itself in the 

 sand, and consequently are very moveable. They are attached to 

 each boss by a circle of muscular fibre, and (in the case of the larger 

 ones at least) also by a ligament passing from the centre of the base 

 of the spine, which is hollowed like a saucer for this purpose, to the 

 top of the boss, similar to the ligamentum teres of the hip-joint. 

 Naturally after death, this ligament, as well as the ring of muscular 

 matter, decays, and the spine falls ofi". For this reason the spines 

 are very seldom found near the echinus in a fossil state, and can 

 only be kept in their proper position in recent specimens by very 

 careful preparation and preservation. 



Coating the whole of this solid part of the echinus, corona and 

 spines, is the fleshy part of the animal, consisting of an apparently 

 structureless mass of gelatine, thus showing a most important 

 difference between this and other animals possessing a similar cal- 

 careous covering, as lobsters, &c., for in the case of the lobster, the 

 hard parts are outside, and when the animal has out-grown its case 

 it has the power of creeping out of it and forming another larger one 

 in its place. In the echinus, however, this is impracticable : the hard 

 part is inside, and hence the necessity for the corona to be made up of 

 separate plates, for each one increases by additions to its edges, and thus 

 the general cavity of the corona is increased to the required amount, 

 in the same manner as the skull is increased in size as the brain grows. 



Now ' the echinus is not only capable of moving at the bottom of 

 the ocean, but is also able to climb up the rocks, and even to walk 



