216 



THE POPULAE EDUCATOE. 



downward, so that all confusion may be avoided. We have the 

 following different series : 



1. The central round opening, which is covered by small 

 movable calcareous pieces, called the apical hole. 



2. On one side of this are the minute crowded holes of the 

 taadreporic plate. 



3. In the five plates which surround the apical hole are the 

 five holes, each of which occupies the external angle of its plate ; 

 these are called the generative pores. 



4. In the five plates which are intermediate to and outside 

 these the ocular holes are seen. 



5. Stretching away in five double tracts are the ambulacral 

 holes. 



6. The large opening below for the mouth and its membrane. 

 We are now in a position to indicate the relation of the soft 



parts of the animal to this protective box. All the above- 

 named perforations have their uses ; and a study of these will 

 teach us almost the whole anatomy of the animal. 



The alimentary canal connects the two largest holes which lie 

 hi the vertical axis of the body. The entrance, or mouth, is in 

 the centre of the wide orifice in the under side, which is covered 

 in by a leathery membrane, with the exception of where the 

 pointed teeth project. The curious beak, composed of five 

 sharp teeth, forms a very effective instrument wherewith the 

 animal can scrape away the soft, calcareous rocks in which so 

 many worms and sea-animalcules bore and hide themselves. 

 It is supposed that the animal swallows chalk and animals 

 together, and lives on the nutritive, organic substances, while 

 the chalk, etc., are passed out again, just as in the case of the 

 earth-worm, great quantities of soft vegetable mould are swal- 

 lowed for the sake of the nutriment it contains in the shape of 

 particles of leaves, etc. The protruded part of the beak, how- 

 ever, gives no idea of the very complex machinery by which 

 these teeth are worked from within. If the leathery membrane 

 be cut round close to the shell and pulled out, it brings with 

 it, or allows to be abstracted, a large and complicated hard 

 framework in the form of a five-sided pyramid, with its base up- 

 ward, and its apex formed by the teeth. This pyramid consists of 

 five jaws, each of which is a frame which sustains the tooth, and 

 has attached to it the muscles which move this tooth in all re- 

 quired directions. The jaw being hollow allows the tooth to enter 

 at its upper broader side, and to pass down in a groove which be- 

 comes closely applied to the tooth on all sides at the lower end, 

 and so holds it in a kind of socket, in which, however, it can 

 move downward, as the tooth is worn away below, and is sup- 

 plied from above. The tooth consists of a curved and flattened 

 bar, ending in a point, and having on its inner side a flange 

 to strengthen it, which flange stands out at right angles to the 

 flattened inner surface of the tooth. The tip of the tooth is 

 of enamel-like hardness, but as you trace it up through the jaw, 

 Ic becomes softer and softer, until it is found to be quite 

 without hard deposit at the part where it protrudes above the 

 jaw. This shows that there is a process of continual renewal, 

 the tooth being laid down as a gelatinous substance in which 

 more and more hard, earthy salts are deposited as it is pushed 

 forward, until it consists almost wholly of these, and is fitted to 

 cope with the hard material to the rasping of which the animal 

 applies it. 



Round the base of the pyramid runs a pentangular muscle, 

 which binds the jaws together. From the outer side of the base 

 of each jaw run muscles to the shell at the sides of the orifice 

 of the mouth. These, when contracted, protrude the teeth all 

 together from the mouth. Other muscles unite the sides of the 

 jaws to one another, and these, when contracted, bring the 

 teeth together. A series of long pieces attached to the centre 

 of the base of the pyramid, gives attachment to muscles, which, 

 running to the shell, have the function of approximating the tips 

 of the teeth. In order to retract the whole apparatus when 

 aching together, or to pull away each separate tooth from the 

 rest when acting separately, a number of muscles (two for each 

 tooth) run from the lower end of the jaw to some calcareous 

 loops or arches, which, standing on the sides of the oral hole, 

 rise up within the shell. 



The jaws, however, are not solely devoted to the sustaining 

 and wielding of the teeth, but are themselves amployed to 

 triturate the food, for on the sides of each jaw which are op- 

 posed to the sides of the next jaws on either hand, are found 

 parallel grooves which transform the surfaces into fine files. 



while the ridges between the grooves, stretching further in- 

 ward than the furrows, form saw-like edges, so that after the 

 food, mixed with hard particles, has passed the tips of the 

 teeth, it can be ground down to a fine pulp by these triturating 

 edges and surfaces. 



The food canal does not run in a straight line from mouth to 

 anus, but, after proceeding a short way as a contracted throat, 

 opens sideways into a wider canal, which, after winding once 

 round the inside of the shell, is bent on itself, and winds round 

 back again, and then delivers at the apical hole. This winding 

 enables the food to undergo a more thorough digestion, while the 

 nutritive parts of the food are dissolved, and either pass into 

 the blood-vessels, which are found in the walls of the intestines, 

 or into the surrounding cavity. It must not be supposed that 

 this long alimentary canal is loose in the box, only attached by its 

 two extremities. If so, it would be liable to become entangled. 

 It is attached by a membrane which lines the inner surface of 

 the shell, and then passes off from this round the alimentary 

 tube, so as to hold it in a loop, or rather fold. This arrange- 

 ment is very general, not only in these, but in the higher 

 animals. 



The holes in the five larger plates surrounding the anal opening 

 are those through which the eggs are extruded (in the case of the 

 female) into the sea-water, so to renew the round of life. They 

 furnish the exits for five separate organs situated just below 

 them. The holes in the alternate plates are called ocular 

 holes, because, through them, a nerve passes to an organ, sup- 

 posed to be an eye. The ambulacral holes and the madreporic 

 holes need a further explanation, which will lead to a description 

 of the locomotive organs of the animal. The locomotive organs 

 of the echinus are of two kinds the soft for pulling, and the 

 hard for pushing. The hard-pushing organs are the spines. 

 These are, no doubt, defensive organs, but they also unite with 

 this function that of locomotion. The spines are, as we have 

 said, set upon the knobs of the outside of the shell. They are, 

 however, movable upon these, so that they can be turned in all 

 directions. To effect this movement without destroying the 

 solidity of their attachment, there is a curious contrivance. At 

 the centre of the concave base of the spine, there is (at least 

 in the purple-tipped sea-urchin) a pit corresponding to the pit in 

 the centre of the tubercle on which it is set. A ligament 

 runs from one pit to the other, and so prevents the spine from 

 slipping off its support, while from the edges of the base of 

 the spine muscular fibres run to the membrane which clothes the 

 shell. It will be seen from this that the shell is not naked, but 

 covered with irritable and live membrane, which membrane 

 passes down between each plate, and, no doubt, subserves the 

 function of secreting fresh matter round the edges of these 

 plates as the animal grows. How far the spines may aid the 

 animal in progression may be a matter of question ; but those 

 who have observed its motion believe they are concerned in 

 it. By far the most efficient organs of locomotion are the 

 little tubular feet ending in discs, which are protruded through 

 the ambulacral holes. These feet act like suckers, when ap- 

 plied to the rock on which the animal moves. The coatings of 

 circular and longitudinal muscles which enclose the hollow tubes 

 are sufficient to move the animal when a multitude of these discs 

 have been extended and attached ; but the question arises, how 

 are they protruded ? This is done by a curious contrivance. 

 Each little tube, after traversing the shell and arriving at the 

 interior, expands into a muscular bag. Both bag and tube con- 

 tain liquid. All the little bags, set on each line of ambu- 

 lacra, communicate with a vessel, which stretches from mouth 

 to anus, and these ten vessels all communicate with a ring round 

 the mouth, which ring has, opening into it, some larger bladders 

 to contain a reservoir of water, and it also communicates with 

 the madreporic holes by a tube, which is filled with fine sand. 

 The method of protruding the tubular feet is supposed to be 

 the following : sea-water is filtered through the madreporic 

 plate and sand canal to the ring round the mouth. When the 

 animal is in a lively state and inclined for locomotion, the 

 bladders force the water into the rows of little bags, and these 

 being muscular, can, by contracting, force out any or all of the 

 sucking feet at pleasure. When, on the other hand, the animal 

 wishes to retract all its feet, the bags, distended by receiving 

 all the water which was in the tubes when extended, would be 

 in an awkward state of tension, unless the fluid were allowed ta 

 pass back into the ring and bladders. 



