544 THE SEA-URCHINS, OR SEA-EGGS. 



We now come to the SEA CUCUMBER, which has received its generic name from its great 

 resemblance to that vegetable. The smaller species are appropriately named Sea Gherkins. 

 The food of all these animals consists of marine mollusks and other small inhabitants of the 

 sea. The complete but empty shells of several small mollusks have been found within the 

 stomach of dissected specimens, proving that the creature must have swallowed the shell 

 entire, and dissolved out its inhabitant by the process of digestion. 



It may as well be mentioned that the only vestige of a skeleton in these creatures is a ring 

 of chalky substance surrounding the beginning of the intestinal canal, and formed of ten 

 pieces, five large and as many small. To this curious ring are attached the longitudinal 

 muscles of the body, by which the creature can lengthen or shorten itself at will, the expan- 

 sion and contraction of the body being due to a series of transverse muscular fibres. The 

 longitudinal muscles are ten in number, and are arranged in five pairs. 



Another example of the Cucumarise is termed Cucumaria lyatina. It is remarkable for 

 its beautiful mouth, which is adorned with a crown of tentacles. 



A Cucumaria called Synapta is a more singular being. It derives its name from a Greek 

 word signifying to seize hold of anything. This name is given to it because, when the hand is 

 drawn over its surface, the skin is slightly arrested by some invisible agency. 



On taking oif part of the skin of the Synapta and placing it under a microscope, a most 

 wonderful sight is disclosed. The skin is furnished with a number of little tubercles on which 

 are set numbers of tiny spicules, which look as if they were anchors for a fairy fleet. They 

 are of extremely minute dimensions, and are quite invisible without the aid of a microscope, 

 but never fail to excite admiration when they are well exhibited. Perhaps the best method 

 of bringing out their beautiful shapes is by using a parabolic condenser or a spotted lens, as 

 then their translucent glassy forms shine out against a dark background. 



These little objects are of exactly the same shape as the classic anchors of ancient times, 

 and were it not for their extreme minuteness, the person who sees them for the first time is 

 tempted to think that they have been manufactured by some ingenious impostor. But the 

 hand of man is quite incapable of making these beautiful little objects, with their long shanks, 

 their gracefully curved arms, and their sharply-pointed and regularly-serrated flukes. 



Nor are the anchors the only wonders which so appropriately deck the skin of a marine 

 animal. If the little prominences can be neatly placed under the microscope without being 

 rubbed, each anchor is found to be affixed by the end of the shank to the end of a curiously- 

 formed shield, made of the same translucent substance as the anchor itself, and pierced with a 

 perfectly regular pattern like ladies' "cut- work" embroidery. These shields hold the anchor 

 in such a way that, as the shield lies flatly upon the skin, the flukes of the anchor are held in 

 the air. The object of this remarkable arrangement is not known. 



There are several species of Synapta, all with the anchors and shields, but the pattern 

 upon the shields is different in the various species, as is the shape of the anchor. These 

 remarkable appendages have been compared by some authors to the little hooks on the calyx 

 of the well-known burdock. Synaptas are abundant on the Reef, and one or more are found 

 on the shores of New England. 



WE now come to a new and beautiful family of this order, called Echinidse, because they 

 are covered with spines like the quills of the hedgehog. Popularly, they are known by the 

 name of SEA-URCHINS, or SEA-EGGS. The general shape of these curious beings can be best 

 learned by reference to our colored illustration, which in every respect is most true to natnre. 



In all these curious beings the upper parts are protected by a kind of shell always more 

 or less dome-shaped, but extremely variable in form, as will be seen in the illustrations. The 

 shell is one of the most marvellous structures in the animal kingdom, and the mechanical 

 difficulties which are overcome in its formation are of no ordinary kind. In the case of the 

 common SEA-EGG, the shell is nearly globular. Now, this shell increases in size with the age 

 of the animal ; and how a hollow spherical shell can increase regularly in size, not materially 

 altering its shape, is a problem of extreme difficulty. It is, however, solved in the fol- 

 lowing manner : 



