Sea urchin

“ A Chilean form and also a South Pacific species of Hemiaster carry their young in brood-pouches, and they also directly develop, for no pluteus sea-urchin larvae were captured by the Challenger expedition in the Southern Ocean. The large sea-urchin of the Mediterranean is an article of food, and the Indians of the northwest coast eat the large local species. ”

“ The sea urchin lays eggs, and from these eggs come the young urchins. (What other animal have we already found, that lays eggs?) When they are babies they don't look a bit like their parents. And here is another thing that the sea urchin does that you will remember when we come to kangaroos. Some of the sea urchins carry their babies in pouches. They fold their spines over their babies just as you carry a doll in your arms. ”

“ Such a regular urchin as has here been described lives with the mouth downwards, preferring a hard floor, on which it creeps by its podia and its radioles, constantly scraping the algae and seaweeds from the rock with its teeth and so feeding itself. If it does not bore a hole, or is not protected by long needle like radioles, it may grasp bits of sea-weed or other objects with its pedicellariae and hide beneath them from the fish that seek it for food.
The Irregular urchins (fig. 3) have been modified for another way of life. Some of them live in mud or ooze, through which they creep. ”

“ It has no head—that is, the development of its nervous system and the direction of its locomotion are not fixed in a forward direction, as is the case with vertebrates and insects.
Some members of the sea urchin family have hoof-like formations on the ends of some of their spines, with which they are enabled to walk over the sea bottom without using the suction disks. About the only enemy of these fearsome nightmares of the deep is man. Some species are used extensively for human food, notably among the Mediterranean coast and in the West Indies. ”

“ Of slender-rayed brittle stars there are brown and green and mottled sorts, some with white cross bands on each arm, and all with the fragile arms breaking away with the least roughness in handling. Often merely the contact with the preserving fluids seems to be sufficient for a general epidemic of arm-shattering. Among the sea urchins a kind with very slender, long, almost needle-like spines is abundant. These spines are not only sharp, but stinging, and often a warning tingle told the exploring hand in crevice or pool bottom of the presence of this well-protected little urchin. ”

“ The habitation of this animal is chiefly in crevices of rocks, the spines projecting on every side, prevent it from falling, and when the sea-urchin wishes to move forward, the tube-feet are thrust out between the spines and fastened to the rock, then by contracting they draw the shell along.
Star-fish and sea-urchins appear to be quite dissimilar in structure, but a little examination and a comparison with allied species will show that there exists between them a close connection. ”

“ They grow in number and size with the age of the sea-urchin, without alteration of the general shape of the shell, since fresh deposits of the calcareous matter that constitute it are made upon the interior edges of each plate.
Sea-urchins are not sociable creatures. They prefer solitude to company, and rarely more than one occupies one dwelling. ”

“ Comparison of Starfish and Sea Urchin.—The water system of the sea urchin, consisting of madreporite, tubes, and water bulbs, or ampullæ, is similar to that of the starfish. The tube feet and locomotion are alike. There is no need for well-developed respiratory organs in either animal, as the whole body, inside and out, is bathed in water. The method of reproduction is the same.
Fig. ”

“ Sea urchins are spherical, oval or flattened free-moving echinoderms covered with spines. Their soft tissue anatomy is characterized by a dominant digestive tract, a varying number of gonads, and the highly specialized water vascular system with its external appendages, the tube or ambulacral feet. ”

“ Although several specimens of the echinoid taxon missing in our study, the Echinothurioida (or leather urchins) , were found in museum collections, their shape prevented successful soft tissue imaging. These sea urchins possess a flexible calcite endoskeleton as well as specialized internal muscles that contract when the animal is fixed, resulting in a pancake-like shape of the organism. ”

“ A specimen in the Museum of Practical Geology, in London, still further prolongs the period which must have elapsed between the death of the sea-urchin and its burial by the Globigeringæ. For the outward face of the valve of a Crania, which is attached to a sea-urchin (Micrastor) , is itself overrun by an incrusting coralline, which spreads thence over more or less of the surface of the sea-urchin. ”

“ As sea-urchins do not shed the shell, as do crabs and lobsters, the inquiring mind will naturally ask how the animal can continue to enlarge when once it is invested with a hard shell. The answer is, that every piece of the shell grows at the same time, and in this way the whole shell enlarges together, and in a perfectly symmetrical manner.
As already indicated, the sea-urchin moves by means of its locomotive suckers. Extending these beyond the spines, it lays hold of the surface of the rock or sea-weed, and then, contracting the suckers, pulls itself along. ”

“ Sea urchins, like their allies the starfishes, generally inhabit deep water beyond low-water mark, where they often exist in enormous numbers, feeding on both animal and vegetable substances; but some species are often to be met with between the tide-marks, where they may be seen under stones, and frequently half hidden in mud. The globular species occur principally on rocky coasts, but the heart urchins are more commonly dredged from banks of sand or mud that are always submerged. ”

“ The Sea-urchins or Echinoidea (Gr. ἐχῖνος, Hedgehog or Sea-urchin) , which constitute the third class of the Eleutherozoa, have derived both their popular and scientific names from the covering of long spines with which they are provided. At first sight but little resemblance is to be discerned between them and the Starfish and Brittle Stars. They are devoid of any outgrowths that could be called arms; their outline is generally either circular or that of an equilateral pentagon, but as their height is almost always smaller than their diameter, they are never quite spherical ”

“ The attachment of the larger radioles is protected by a ring of smaller ones. These and the other small spines protect the sea-urchin, as its prickles protect a hedgehog; the larger ones may also help the animal to move or to fix itself firmly against the shock of waves. Some urchins, especially the purple egg-urchin, bore holes even in very hard rocks, and by stretching out their radioles they can hold themselves immovably in their holes ”

“ Fig. 116. Masticating apparatus of Echinus lividus.
Nothing is more curious than to see a sea-urchin walk upon smooth sand. But for the colour, it might be mistaken for a chestnut with its bristling envelopes, the spines serving as feet to put the little round prickly mass in motion. They have even been observed to form themselves into a ball, and roll along like a globular fagot of prickles.
One of the most singular organs of the sea-urchin is its mouth. It is monstrous. ”

“ Distinct features of internal sea urchin anatomy may extend to the sizes and shapes of digestive tract, gonads, ampullae and muscles. A thorough revision of internal structures in the light of recent technological advances will therefore almost certainly change our view of facts that, for years, were taken for granted. Scanning of sea urchins has so far been performed by us on more than 50 species (Ziegler et al., publication in preparation) . ”

“ Sea-urchins live only in the sea, from between tide-marks down to all but the greatest depths. The abyssal forms have very thin tests, which are often flexible. Urchins eat all kinds of animal and vegetable food, and are themselves attacked by fish, by star-fish, and even by other urchins. The ripe egg-bunches area favourite article of diet with dwellers round the Mediterranean; in other respects sea-urchins are of small importance to man, being neither useful nor harmful. In olden times the larger radioles were recommended to be powdered and taken as a remedy for the stone. ”

“ We have seen that the Sea-Urchins approach most nearly to the sphere, and that in them the oral region and the sides predominate so greatly over the ab-oral region that the latter is reduced to a small area on the summit of the sphere. In order to transform the Sea-Urchin into a Holothurian, we have only to stretch it out from end to end till it becomes a cylinder, with the oral region or mouth at one extremity, and the ab-oral region, which in the Holothurian is reduced to its minimum, at the other. ”

“ In the neighbourhood of Torone there are sea-urchins of a white colour, shells, spines, eggs and all, and that are longer than the ordinary sea-urchin. The spine in this species is not large nor strong, but rather limp; and the black formations in connexion with the mouth are more than usually numerous, and communicate with the external duct, but not with one another; in point of fact, the animal is in a manner divided up by them. The edible urchin moves with greatest freedom and most often ”

“ This is the Hemiaster sea-urchin. She lays but a few eggs, and these she jealously guards in a number of pouches on her back. Here they hatch, and in due time become young sea-urchins (fig. 2) . One of the starfish, again, carries its young on its back under a wonderful tent stretched across the tips of specially constructed spines; and, in order that water may constantly reach her family, the roof of this tent is pierced with holes! Even the unsightly sea-cucumber, or sea-slug, is not to be outdone. ”

“ If, now, we compare these animals in the various attitudes natural to them, we may fail to recognize the identity of parts, or, at least, it will not strike us at once. But if we place them all—Holothurian, Sea-Urchin, Star-Fish, Ophiuran, and Crinoid—with the oral or mouth side downward, for instance, we shall see immediately that the small area at the opposite end of the Holothurian corresponds to the area on the top of the Sea-Urchin; that the upper side of the Star-Fish is the same region enlarged; that, in the Ophiuran, that region makes one side of the small circular disk ”

“ Now the series of events here attested by a single fossil may be carried a step farther. Thus, for example, we often meet with a sea-urchin (Ananchytes) in the chalk (see Figure 12) which has fixed to it the lower valve of a Crania, a genus of bivalve mollusca. The upper valve (b, Figure 12) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, then died and lost its spines, which were carried away. ”

“ Examine a live sea urchin, or the common five-rayed starfish, in a rock pool or aquarium, and it will be seen to possess a large number of soft, flexible, and protrusible processes, each of which terminates in a little sucking-disc that enables the animal to obtain a good ‘foot-hold;’ and, having fixed itself on one side by means of a number of these little ‘feet,’ it is enabled, by the contraction of certain muscles, to pull itself along. ”

“ These spines, however, are very easily broken off, and when the animal dies, and its empty shell is tossed to and fro by the waves, they are knocked off in a very short time; so that when you meet with a sea urchin’s shell lying upon the shore you nearly always find that it is covered with nothing more than hundreds of very tiny pimples. ”

“ If it wishes for change of place, the anterior filaments contract themselves, whilst the hinder ones loosen their hold, and the shell is carried forward. The sea-urchin can thus advance with ease, and even rapidity. During his progression the suckers are only slightly aided by the spines. It can travel either on the back or stomach; whatever their posture, they have always a certain number of prickles, which carry them, and [286] suckers, with which they attach themselves. In certain circumstances the animal walks by turning upon itself, like a wheel in motion. ”

“ The wind having fallen on the 29th, and left a practicable channel, we at last doubled the rugged and rocky Cape Barrow. On the top of rocks, upwards of a hundred feet aboye the sea, clam-shells were found, and some small specimens of that round prickly sort of shell called the "sea urchin," which must have been carried by birds to such an elevation. These, with cockles, muscles, periwinkles, and seaweed, abounded here, and in many other places on the beach. ”

“ If a living Sea-urchin can be procured, and placed in a glass vessel filled with sea-water, it will at once be seen that{316} its surface is thickly covered with spines. In some species these spines are as thick as ordinary drawing pencils; but in most of those which are found on our shores they are very slight, and scarcely longer than darning-needles. They are in almost perpetual motion, and generally have a sort of revolving movement, the base being the pivot. ”