62 THE SEA URCHIN. 



its spines, the creature is enabled to move from place to place. 

 Using a pocket lens, we may also see attached to the spines 

 and scattered over the surface of the creature, curiously- 

 shaped appendages, which for years have been a great puzzle 

 to naturalists. These are the pedicellarice, but of these and 

 the sucker feet more anon. 



Let us now turn our attention to the " test," or as it 

 is inaccurately called, the shell of the creature, as we see 

 it divested of the spines. It is in reality the limey skeleton 

 of the animal, and not an outer coat of armour like the shell 

 of crab or lobster. It is not made in one solid piece, but 

 formed of numerous pentagonal plates (over GOO in a single 

 test) composed chiefly of carbonate of lime, which the animal 

 has the power of secreting from the sea- water where it is held 

 in solution. These plates are fitted together with the most 

 marvellous accuracy, and over the whole surface, and between 

 each several plate is stretched a thin film of living flesh, so 

 fine and colourless, indeed, that the microscope alone reveals 

 its presence. And herein is a marvel indeed ! The Echinus 

 when it reaches the last stage of its development assumes 

 the shape it retains through life. Hence the same skeleton 

 encloses the young as the aged urchin, for, of course, it does 

 not shed its skeleton as the lobster does it shell. Gradually 

 the limey particles are secreted and deposited along the edges 

 and surface of the many plates in a determinate manner, and 

 with the utmost regularity. They grow with the creature's 

 growth and strengthen with its strength, and still the same 

 shape is preserved that was first improved upon it. The whole 

 case or shell consists of five zones or segments, in which are 

 double rows of pentagonal plates perforated with double rows 

 of pores, and between these five other zones of about twice 

 the breadth. The former are generally called Ambulacra 

 (L. ambulacrum, a walk), being fancifully considered to 

 resemble the walks of a garden. The surface of the plates 

 in the larger zones and also those between the perforated 

 ones is studded with warts or tubercles of different sizes 

 arranged in rows from pole to pole. The upper part of each 

 tubercle looks like a highly polished billiard ball. To this 

 polished portion the spine is attached, the base of the latter 

 being hollowed out to receive it, and thus we have a perfect 

 " ball and socket " arrangement. The character of the spines 

 varies much in different species ; in some they are fine and 

 silky, in others stout and club-like, in some straight and rigid, 

 in others curved and flexible. The growth of the spines is 

 curious, and may aptly be compared to that of an exogenous 



