Chapter II: Front Matter (2)

The Apiocrinidæ attained their maximum during the Jurassic period, when they were represented by many fine species of the genera Apiocrinus and Millericrinus. The chalk genus Bourguetticrinus shows many symptoms of degeneracy. Rhizocrinus loffotensis, M. Sars, was discovered in the year 1864, at a depth of about 300 fathoms, off the Lofoten Islands, by G. O. Sars, a son of the celebrated Professor of Natural History in the University of Christiania, by whom it was described in the year 1868. It is obviously a form of the Apiocrinidæ still more degraded than Bourguetticrinus, which it closely resembles.

The genus Bathycrinus must also be referred to the Apiocrinidæ, since the lower portion of the head consists of a gradually expanding funnel-shaped piece, which seems to be composed of coalesced upper stem-joints. The stem of Bathycrinus gracilis is long and delicate; in one example of a stem alone, which came up in the same haul with the one nearly perfect specimen which was procured, it was 90 mm. in length.

The general distribution of the deep-sea Asteridea has already been referred to. Perhaps the most obvious peculiarity which they present is the great preponderance of the genera Astrogonium, Archaster, Astropecten, and their allies. Genera belonging to other groups do not apparently become less numerous, for species of Asteracanthion, Cribrella, Asteriscus, and Ophidiaster are as abundant as they are at lesser depths; but as we go down new species with tessellated mailing on the disk and massive marginal plates seem to be perpetually added.

Of the twenty-six Echinoderms dredged from the _Porcupine_, seven--Porocidaris purpurata, Phormosoma placenta, Calveria hystrix, C. fenestrata, Neolampas rostellatus, Pourtalesia jeffreysi, and P. phiale--are forms which have been for the first time brought to light during deep-sea dredging operations, whether on this or on the other side of the Atlantic. There seems little doubt that these must be referred to the abyssal fauna, upon whose confines we are now only beginning to encroach. Three of the most remarkable generic forms--Calveria, Neolampas, and Pourtalesia--have been found by Alexander Agassiz among the results of the deep dredging operations of Count Pourtales in the Strait of Florida, showing a wide lateral distribution, while even a deeper interest attaches to the fact that while one family type, the Echinothuridæ, has been hitherto only known in a fossil state, the entire group finds nearer allies in the extinct faunæ of the chalk or of the earlier Tertiaries than in that of the present period.

Many of the mollusca from the deep water have hitherto been found only in the northern portions of the area examined, and are generally allied to northern forms.

The abyssal mollusca are by no means devoid of color, though, as a rule, they are paler than those from shallow water. Neither are the abyssal mollusca universally destitute of eyes. A new species of Pleurotoma from 2,090 fathoms had a pair of well-developed eyes on short foot-stalks; and a Fusus from 1,207 fathoms was similarly provided. The presence of organs of sight at these great depths leaves little room to doubt that light must reach even these abysses from some source. From many considerations it can scarcely be sunlight. The whole of the light beyond a certain depth might be due to phosphorescence, which is certainly very general, particularly among the larvæ and young of deep-sea animals.

“THE MIMIC FIRES OF OCEAN”
--G. CLARKE NUTTALL

Nature dazzles the eye of man with many wonderful phenomena, but perhaps never more so than when she turns the gloomy night waters of the sea into a sheet of silvery fire. At these times every movement of the wave, every cleavage of the water by oar or prow, reveals in its dark depths a hidden fire which scintillates and sparkles with weird and mysterious light. The spectacle is one of absolute fascination, for the Spirit of Enchantment rests upon the waters and reality becomes fairyland.

The ancients, keenly alive to a sense of the supernatural, saw in this luminosity a manifestation of some unknown power, and wondered; the ignorant read in it a portent of judgment and terror; while in all ages the curious and the searchers after knowledge have speculated as to its cause. But just as Nature has invested its appearance with a halo of mystery, so she has also wrapped in much obscurity its immediate cause; and thus, though in the course of centuries varying suggestions have been put forward, nothing with any finality about it has been arrived at. It was asserted truly that certain fishes were luminous; sharks have glowed and shone, shoals of herrings, pilchards, or mackerel have been moving masses of light, and the fish drawn out of the water have lain in great shining heaps, the glow of which vanished as they dried and died.

Many writers have described the passages of ships through such shoals--the sheet of moving flames--the beautiful pale greenish elf-light that the fish exhibited; while poets have apostrophized the “mimic fires of ocean” and the “lightnings of the wave,” and scientists and naturalists have in turn tried to account for their power of luminosity. Some have attributed it to the presence of certain substances of a fatty nature excreted by the fish and adhering to the surface of their bodies; others have declared that it is due to a subtle power of the fish itself--a form in which the energy of life shows itself under certain conditions, just as this energy may be exhibited in heat, or motion, or electricity; others, again, have ascribed it to direct absorption and transmission of the light of the sun, and so on. Many theories have been elaborated, but none convincingly.

But now, it is asserted, the secret is laid bare.

It is wonderful how many secrets the searching light of the Nineteenth Century is claiming to reveal.

It is only lately that any very serious effort has been made to study this phenomenon, but the research has been abundantly rewarded, for it is now pretty certain that the luminosity is due to the presence in the water of various kinds of bacteria.

Now, bacteria are the very smallest living organisms of which we have cognizance. Millions of them can lie on a penny; therefore, to produce the gleaming appearance recognized by us as phosphorescence, they must be present in numbers too enormous even to contemplate with our finite minds. It would be immeasurably easier to reckon with the stars for multitude than with these phosphorescent bacteria. They are colorless, rod-like bodies, only known to us in the land revealed by the highest powers of the microscope, and careful comparison shows minor differences among them. For instance, some of them are capable of independent motion--we can hardly call it swimming--others are non-motile, some are inclosed in a jelly-like covering, others are without this sheath. Their power of motion is probably due to excessively fine hairs at their extremities, which, moving to and fro in the water, act the part of oars. These cilia have not been found in all forms of bacteria which move, but their presence is inferred, since every advance in the study of motile forms increases the number of bacteria which are seen to possess them.

These light-producing bacteria are known as photo-bacteria, and so far some half-dozen varieties have been distinguished and named. The names in such cases are usually either given from the locality of their appearance (thus, photo-bacterium Balticum, found in the Baltic), from their discoverer (for example, photo-bacterium Fischeri, after Professor Fischer), or from some striking attribute (to wit, photo-bacterium phosphorescens, the commonest light-giving species).

A Dutchman named Beyerinck has made a special study of these photo-bacteria, and has experimented with them in a great number of ways to determine, if possible, why they should thus become illuminated, and if the light plays any notable part in their life-history; but his results are, seemingly, all more or less of a negative nature. He can not find that it has any very important function. The breathing of these tiny organisms is not, apparently, in any way bound up with it; their nutrition, growth, and development go on quite well even if they are placed under such conditions that their luminosity is arrested; in no way, indeed, is it a vital process. It only seems to depend on the food which the bacteria feed upon and the presence of oxygen. Given suitable food and plenty of fresh air, and they exhibit their characteristic light; deprive them of one or the other and they no longer shine.

This knowledge helps us to understand, then, the phenomenon of phosphorescence. It is visible only at night because in the full glare of day the greater light overpowers the lesser; it is visible at certain times and seasons because the conditions are such as to evoke it. And what is favorable for the lighting up of a single bacterium is favorable for all; hence the myriad multitudes of infinitesimal units, each set glowing with its tiny light, are sufficient in the sum total to put a whole ocean aflame.

It would, of course, be presumptuous, and doubtless erroneous, to say that all the phosphorescence of the sea is due solely to photo-bacteria; it can only be asserted in the present state of our knowledge that they are certainly responsible for a great share of it. But this wonder of nature must now be regarded as yet another instance of the mighty results accomplished through the agency of the smallest of living things.

THE JELLY-FISH AND OTHER HYDROZOA
--P. MARTIN DUNCAN

If anybody were asked to name the commonest objects on a sandy or pebbly seashore, the reply would be--sea-weed, crabs, limpets, a stranded jelly-fish, looking like so much blubber, and a sea-gull flying overhead. The first three things really belong to the shore; the bird is a visitor that is looking after food, and the large, turned-upside-down, basin-shaped, jelly-looking mass is a waif from the ocean. If one of these stranded jelly-fish is looked at, as it lies on the sand, its shape appears to be something like that of an umbrella without the stick. It is thick, however, and has some curious markings about its surface and inside. Even something like a fringe of thick hairs may be detected around the edge. On trying to take it up or turn it over, the fingers go into it, so readily does the animal tear; but on getting the umbrella on its back, the underneath part is seen to be made up, in the middle, of some projections slightly more solid than the rest of the body; and if put between them, the finger passes through a sort of tube into a cavity in the body, which is the stomach.

Some of these jelly-fish, or Medusæ, as they are called, are pounds in weight; but after a while only a shred of membrane-looking stuff remains, the water which makes up nearly all the animal having drained away. It is evident that the large Medusæ grow from small ones, and that there are many kinds of them, some of which are always small. A great jelly-fish, two feet across, was once a little one, leading exactly the same life. They grow, and to grow they must have nourishment; they are never still, and their muscles, transparent as they may be, have to be nourished, or their strength would soon give way. They require not only food, but also air in the water, so that they consume it, and make its oxygen gas of use to them. They seek the light in a remarkable manner, and get out of the way of things very gracefully, and their motions are rhythmical, like the ticks of a clock, in succession. Have they nerves and eyes? Science answers in the affirmative. These creatures, consisting of a vast proportion of sea-water, breathe, digest, and feed. More than this, they produce young; and if they are pale in color, bluish, or roseate in hue during the day, they are the glory of the deep during the night; and each one is a globe of light, the luminousness being the result of the action of the mysterious energy of life upon matter.

Many of the jelly-fish found on the shores dead and injured are eighteen inches across, and it is not uncommon to see one swimming freely whose body is larger than that. Yet in spite of this size, and of the gifts of the creature, it has one of the shortest of lives, and it is born, grown, and dead between the spring and the winter.

1, Rhizosphæra; 2, Sphærozoum; 3, Actinomma; 4, Lithomespilus; 5, Ommatocampe; 6, Carpocaium; 7, Challengeron; 8, Heliosphæra; 9, Clathrocyclas; 10, Dictyophimus]

On examining a large jelly-fish, it will be noticed to have not only four round bodies on the top, round the centre, and the four lobes hanging down beneath, but that the edges of the body are not quite round, but are notched, so as to make eight lobes to it. And if a little care be taken, marks can be seen on the under part of the body, from the round centre spots to each of the eight splits in the disk; and a magnifying-glass shows a little substance there which feels gritty, and is sometimes colored. Other branching tube-marks pass from the midst of the body to the edge of the umbrella, and a tube runs all round the edge. The tubes communicate in the midst of the body with a cavity, into which the finger can pass between the four under lobes--the stomach--and the digested matters pass into them to nourish the creature. Around the disk or umbrella, and outside the circular tube, is the fringe of hairs which have, to a certain extent, to do with the capture of prey. Each consists of a filmy substance, in which are fixed thread-cells, not very unlike those of the sea-anemone, but they have longer barbs and sharper thorns stretching out from them. Any violence or irritation causes the thread to shoot forth and to injure. The rest of the body of the Medusa is covered with an excessively thin skin, which has movable cilia upon it. It is supposed, and with some reason--for nervous threads and bodies have been found on the disk, close to the gritty bodies especially--that these last are eyes, or ears, or both. The nerves supply the muscles that move the umbrella, and cause it to expand and contract, and the muscles are remarkable, for some are not simple contracting fibres all made of one piece, as in the Anemone tribe.

The jelly-fish, when in full vigor and weighing many pounds, must catch and eat much, and the manner in which this is done is by no means perfectly understood. The food is digested, and the results are taken from the stomach into the numerous tubes in the body, where they nourish the tissues. The movements of the animal and its extremely delicate membranes enable much water to come in contact with it, and it breathes by that means, for there are no special gills or lungs.

The four round spots on the top of the body, sometimes white and sometimes red in color, are spaces in the body, and they open into the central stomach. The circles of these chambers are lined with ciliate membrane, and in some places it is arranged in folds, and in them the eggs or ova form. Little oval or roundish things, like pins’ heads are they: they burst forth and pass out of the stomach, and then through the canal into the four lobes hanging down like pouting lips. Here they rest a while until a little grown, and they start out on their journey of life in the autumn time as little oval or long things made up of cells, the outside ones having movable hairs also on them. Totally unlike the parent, the tiny offspring floats off with the tide and does a little work in moving itself with its hairy covering. Sooner or later this minute thing, which is called a planula, settles on a stone or piece of sea-weed, and begins to grow. First of all its outer skin is formed into a hollow on the top, a kind of basin-shaped cavity being produced. This is the future stomach. Then some little projections grow around the hollow, and stick out in the water like so many rays, and they increase in number and length very rapidly. Under this shape the creature has been described as a hydra or a polype, and it really resembles a small sea-anemone with very long and slender tentacles. But the internal anatomy differs. The creature is all stomach and tentacles, and it grows by catching small prey. All the elaborate tissues of the jelly-fish are absent and the creature can not move. It is this curious condition of life that brings the jelly-fish tribe within notice of a seashore observer, for those little hydras live in some places at low-water mark. The knowledge that the hydra was the child of the jelly-fish was obtained many years ago by a Scotch and by a Scandinavian naturalist. It was known that some small kinds of jelly-fish which were kept in sea-water in an aquarium disappeared altogether in the late autumn. Then it was noticed that there were many very small things with tentacles, on the sides of the glass, which had not been noticed before. And at last the curious discovery was made that these hydras or planulas reproduced jelly-fish the next year. This is a very strange story, and the course of nature’s proceeding is, that when food has become scarce and the boisterous waves would become fatal to the delicate Medusa, it shall lay eggs which produce creatures that can settle down out of the way of the rush of water, and that require but little food. Then in the next year when food is in plenty, the progeny separate into jelly-fish like the parent. The hydras or progeny have no power of laying eggs; they digest and produce the creature that shall lay eggs. The Scandinavian naturalists and Germans called this “alternate generation,” and named the hydra the nurse of the jelly-fishes. This is the method by which most of the jelly-fish, which may be occasionally seen stranded on the coast, were produced; but there are some whose planulas do not settle down, but are always free swimmers, and they divide into Medusæ in the sea.

This intensely interesting course of life is modified in all the hundreds of kinds of creatures which are connected with the jelly-fish in classification, by their having many structures in common. The creatures thus classified with the jelly-fish are called Hydrozoa, or Water Animals, and they pass two lives, one of which is commonly observed on the seashore everywhere. One life is a fixed one, and the other is a free swimming one; in one stage it is eating and drinking, and in the other, these functions are not always or often carried out. The egg-laying and perpetuation of the animal are the duty of the greater part of the beautiful branched and hair-like things which are arranged by visitors to the seaside in patterns, and retained as memorials of pleasant days, and which are popularly called sea-weeds. They are not such things, but are the delicate stems, branches, and bud-like homes of the parents of tiny jelly-fish, which are only to be caught in the open sea. These horny stems and branches end in creatures with tentacles or feelers, and they live between tides, in rock pools, and at low water.

Sometimes these fragile, plant-looking things are fixed on to stones or sea-weed, and some get a ride by growing on shells, inhabited, since the death of their original possessor, by the Hermit Crab.

One very numerous tribe of these feathery-looking weeds, but which are truly animals, is common everywhere on the shore, between tide-marks into deep water. Their stems are branching, and the little bud-like things on them, when watched in still water, may be seen to put forth pretty colored bodies with tentacles around a centre; and as there may be hundreds of them on the branches and stem, the creature resembles a bunch of rayed flowers. Hence the name Sertularia, from sertula, a small garland.

These Sertularians were noticed and drawn many years ago, and were called Corallines. This is a great mistake, because the Coralline is a plant covered with carbonate of lime. The correct name would be the Garland polypes. When we get a bunch of these creatures from the seaside they are dead, and the hard and preservable outside parts alone are left, all the beauty of color and the wonderful inside structure are gone. But even then it can be noticed that the stems of the creature arise from a network of tubes fixed on to stones, sea-weeds, and shells. This resembles a root, but it does not absorb nourishment like the root of a plant. The stems, often not bigger than hairs, are hollow, and the branches also. The surface of the branches is covered on one, or often on both sides, by minute cups. These give almost a saw-edge look to it, in some of the creatures; and among these cups, which are open at their free end, are some larger ones, which are closed where free, and often ribbed and ornamented on their outside. It is noticed that the hollow of the stem and branches is continued into the cups, but not into the closed ones, and that these are shut off from it by a very delicate layer of tissue. This is the minute structure of these things when dead; but when alive, the inside of the stem and branches is filled with a soft substance, which reaches up to the part where each of the cups is attached to the outside. The cups, hundreds in number, on each stem and branch, contain a most beautiful flower-like polype. Variously colored, according to the kind, it has a bell shape, and has a spot for the mouth, which leads to a stomach, whose floor is connected with the pith of the whole branch. Around the mouth are numerous tentacles excessively irritable, capable of stinging and paralyzing prey; and the spectacle of hosts of these, all working for the common good of the branching animal, is very beautiful. Sometimes there is much red in the color, at other times green, yellow, or a dull tint; and in order to relieve the sameness of outline presented by thousands of cups, their horny margin is spined or toothed. In full vigor during the summer, and living through the winter and probably for several years, these cups with their tentacled polypes contribute to the growth and nutrition of the whole. They all work for a common end, and that is for the persistence of the life of the colony. They live on microscopic things in the water, such as minute ova, or the young of things like unto themselves, and on animalcules, and probably on the moving spores of sea-weeds. As the spring advances, the larger and closed cups begin to grow by budding from the stem, and they become filled with a gummy substance with a few granules in it, and somewhat resembling the pith of the stem. As the closed cup grows, it often becomes marked on the outside with rings or belts, and it becomes a very prominent object on the horny-looking Garland polype. These closed cups are concerned in the reproduction of the creature, for after a few weeks the substance inside them is seen to collect into bunches of round or oval bodies, often yellow in color, or of other tints. By and by the cup bursts, or a sort of trap-door opens at the top, and these things escape. They are small, covered with cilia, or moving hairs, and resemble the planulas of the jelly-fish in shape, and they swim freely, and finally settle down on a stone or weed. They become environed by a horny coat, the stomach having been formed first by simple bending in of the outside of the creature, and tentacles grow. The planula thus founds a new colony. But there are other phases in this curious life-story; for instead of the little round balls in the closed cups getting free as planulas, some in certain kinds grow there, and resemble excessively minute jelly-fish stuck fast by the back, and exposing the mouth, feet, and the fringed umbrella. These Medusæ, as they may be called, die on the parent, and never wander.

There is one kind of these Garland polypes called Lafœa in which the round bodies produced in the closed cups burst forth, and not in the shape of planulas. They come forth like tiny bells, furnished with minute eye-spots on the edge, and they have an umbrella shape, and long tentacles arising from the edge near the eye-spots. They are jelly-fish, to all intents and purposes, and swim freely about, and in time produce planulas which develop into a parent Garland polype again.

There is a very interesting kind of these allies of the sea-jellies which, from its feathery shape, has been called Plumularia. Specimens are common, and there is a stem, and it has branches, and the cups are in one row on the branchlets. Moreover, there are openings in the horny envelope of the soft pith of the branches, which do not give exit to a polype-looking thing with its mouth surrounded with tentacles, but to a simple pith with many threads or stinging cells in it. It is more or less like a long tongue, and is placed among the cups with polypes, and it is evidently an instrument for capturing minute creatures. The Sickle-beard is one of them.

But one of the most singular of these delicate creatures lives in very numerous colonies, on such an unexpected place as the outside of the shell tenanted within by a wandering hermit-crab. It is called Hydractinia. The Hydractinia has a kind of crust for its formation, with tubes in it, and out from them comes a white film, which, under a magnifying-glass, presents a number of stems and polypes. Each one has at its free part a club-shaped end, which has several rows of tentacles around it. These, often twenty-five in number, are, like the stem, very irritable, flexible, and are covered with a sticky matter. The club is hollow within, for the stomach, and food gets in by the top, where the horny skin appears to be absent.

The tentacles are rather sharp, and when they are half-contracted they often appear to have a knob at the end, otherwise they are stuck out and are like so many thick threads. The stalk which supports the conical-shaped head has a thin and wrinkled skin with sharp, dot-like points on it, and probably they secrete a sticky substance. The polypes thus formed cluster together like so much moss on the shell, and it would appear, if not in the English species, certainly in one which frequents the American coasts, that some groups are male and others female. But there is no doubt, from the researches of Mr. Moseley, F.R.S., of the _Challenger_, that some of the polypes act as catchers of food, and feed those which pay especial attention to increasing and multiplying. These last have minute little knobs growing on the stalk beneath the head, and which develop round bodies inside. After a while motion is noticed in them, and there is every reason to believe that they burst forth and swim off as small jelly-fish. Some, however, give out little planulas, which settle down and grow like the parent.

It will have been noticed from this description that these polypes are different in shape from the Garland polypes and that instead of being bell-shaped, with tentacles around the mouth, they are club-shaped, and have more than one row of tentacles half way down. This last shape gives a characteristic to many little polypes which lead, like the others, two lives--one fixed on the seashore on plants and stones, and the other, freely swimming, as a pretty little jelly-fish.

Another kind lives fixed on sea-weeds in shallow water, and its stem consists of a single tube, which is creeping and threadlike. The cups for the polype are on long stalks, with markings like rings on them, and are bell-shaped with a toothed edge. The mouth is surrounded by tentacles, as in the Garland polype, and, in fact, there is not much outside distinction between them and this Campanularia, or Bell polype.

There is a stomach in the polype bud which leads to a canal that goes down the stalk, and even into the creeping stem or root. The buds, on stalks of their own, are very remarkable. They contain, soon after they are formed, numerous little jelly-fish attached to stalks. There may be twenty or thirty of these in each bud, and the day will come when they will burst forth, be cast loose from their stems, and swim off. They resemble hand-bells of a very flat kind, and the mouth is prolonged into a four-parted projection, which protrudes through a structure not seen in the great jelly-fish, but peculiar to these smaller ones, which fills up the disk underneath, from the mouth to the edge, with a layer of muscular fibres. They have four long tentacles, and when they have lived for some time they begin to play some very curious tricks. Thus a foreign naturalist, M. Van Beneden, was examining some creatures in his aquarium, and found hundreds of these very small jelly-fish, and he caught some in order to examine them with his microscope. He began to draw some of them carefully in order to write a description of them. About an hour afterward, on again looking at his specimen, he was amazed to find its shape changed, and the animal apparently turned inside out. The tentacles on the edge seemed to be reversed in their position; the umbrella-like dome, from being convex was the reverse, and the curled lip-like proboscis seemed converted into the stem of a solitary polype. One of these Campanularians, called erroneously the Wrinkled-thread Coralline, grows on sea-weeds near low-water mark, and especially on the great Riband Tangle. It is a small thing, about an inch in height, and its stem is of a pink or rose-red color; it is sparingly branched in a zigzag manner, and its stem is ringed. The buds develop jelly-fish, but they never escape, and hang on by stalks for the rest of their lives.

FISHES
--ANDREW WILSON

This division every one must know as that of the Vertebrata, a word which may be used in a popular sense, as corresponding to the expression “backboned” animals. At the head of this group man and quadrupeds are found, while the fishes form the lowest class in the division.

There are few groups of the animal world more interesting to the ordinary observer than that of the fishes. To survey the various forms and shapes presented by these animals as displayed in a great museum should prove a sufficient incentive to gain a more intimate acquaintance with the class; and when, even in a popular sense, we investigate the structure and habits of fishes, the study increases in its fascination and interest. While if we reflect that on a knowledge of the habits of fishes, of their distribution in our oceans and seas, and of the special products which many of them offer for our use and luxury, the commercial success of our fisheries depends, it can need no further argument to convince us that, after all, there is something of great practical benefit to be derived from the study of zoological science.

It is not our intention at present to say anything regarding the commercial or economic aspects of fishes, and even their general habits must be very briefly touched upon. We rather aim at giving some account of the structure of the fishes, and at noting such peculiarities in their habits and life as may prove most interesting to our readers. Primarily, then, we find that fishes may be recognized by having the body usually, but not always, covered with _scales_, of various forms and kinds. Then, secondly, we have the limbs represented by certain _fins_; and, thirdly, we find almost all fishes to breathe by _gills_ during the whole of life. These three points are, in the main, sufficient to distinguish fishes from their higher as well as their lower neighbors. The scales which cover the bodies of fishes present great diversities in shape, size, and appearance. Some fishes thus exhibit an utter want of scales; while others, like knights of old, are incased in a veritable suit of scaly armor. The lampreys, and their curious neighbors the hag-fishes, are destitute of scales; and in our familiar eels, the scales are very small and insignificant. Such fishes, however, are amply compensated for the want of scales by the power they possess of throwing out from the skin a vast quantity of glutinous or oily matter, technically named _mucus_. The presence of this secretion, which has given origin to the phrase “as slippery as an eel,” serves to protect the surface of the body, and no doubt also assists in the easy progress of these fishes through the water. So large is the quantity of this oily matter which the hag-fishes can emit from their body, that one form has received the specific name of glutinosa; the fish being able in this way to literally convert the water of the vessel in which it is contained into a jelly-like mass. The familiar blennies, found in rock-pools after the tide has receded, are also able to emit a large amount of this glutinous fluid.

Illustrating the opposite extreme of the development of scales, we find such fishes as the bony pikes of North American lakes and rivers, the bodies of which are covered with an armor of closely fitting and overlapping scales or plates, named ganoid, from their shining appearance (Greek _ganos_, splendor). The scales of this fish are said to be employed in the manufacture of the little “mother-of-pearl” buttons, so commonly used. Many fossil fishes were also abundantly provided with these hard, bony plates; and our living sturgeons possess scales of similar nature, although in the latter fishes they do not completely cover the body. The bright silvery scales of the herring and its neighbors are thin structures, and are very easily detached from the skin; and a curious form of scale is seen in the perches; the hinder edge of each scale in the latter case being cut into comb-like teeth. In the sharks, skates, and rays, the scales are small and horny, and are often provided with little spines. If we draw our hand along the back of a dog-fish from tail to head, as when we stroke a cat’s back the wrong way, we feel numerous small projecting points, borne on the scales. The rough skin surface thus produced is frequently used under the name of “shagreen” in the manufacture of spectacle-cases and like articles, and is also employed for polishing the surface of wood.

In their general shape the bodies of fishes exhibit a great compression from side to side, a rounding of the sides, and a pointing of either extremity, adapting the animals for easy progression through the water. Some fishes, such as the soles, flounders, plaice, etc., are named “flat fishes” from the great flattening exhibited by their bodies; although, at the same time, it is important to observe that these fishes are simply more compressed from side to side than their neighbors. Most persons, on looking at a sole or flounder, are apt to think that one of the flat surfaces must represent the back, and the other the under surface of the body. This idea is strengthened by the fact that the so-called back surface is dark, and the apparent under surface light in color, and because both eyes exist on the dark-colored surface. That, however, the flat surfaces are really the _sides_ of the fish may be seen by noting that on each surface a breast-fin is developed; these fins being placed invariably one on each side of the body. And while the eyes in early life are disposed one on each side of the head, in the position in which eyes are naturally situated, they are gradually brought round to one side by the bones of the head becoming curiously twisted in the course of development. Thus these fishes lie and swim on one side--that which is light-colored--and present a most singular combination of curious and abnormal features.

The fins of fishes constitute interesting features in their structure. Almost all fishes have two sets of fins--those which exist in pairs and those which are unpaired, and which are developed in the middle line of the body. To the former class belong the two pectoral or “breast-fins” and the two ventral or “belly-fins.” The “breast-fins” correspond to the forelegs of other animals or to the arms of man; while the ventral fins correspond to the hindlegs or to man’s lower limbs; and these latter fins may be placed, as hind-limbs should be, to the rear of the body (as in sturgeons); or they may be found (as in the cod) placed beneath the breast-fins on the throat.

It may be asked, How do we know that these two pairs of fins represent the limbs of other animals? We reply, because when we investigate their structure we find them to be supported by a bony skeleton, the various portions of which correspond to those existing in the skeleton of the limbs of man or other vertebrates. And it is only through this important principle of tracing out what are known as the homologies or resemblances between parts, and by looking at and comparing their structure, that we are enabled to find out the real nature of many organs in animals; similar organs frequently existing under very different and varied guises.

The other fins of fishes do not exist in pairs, but are placed in the middle line of the body. Hence they are named the median or unpaired fins. Thus we find the back or dorsal fins to represent the unpaired fins, as also do the tail and anal fins; the latter being placed on the lower surface of the body. These unpaired fins, if they correspond to any other structures in the fishes, are simply to be regarded as special developments of the skin, and therefore bear no true relationship to the limbs of other animals. We may find one or more dorsal and one or more anal fins; but the tail-fin, by the action of which, as every one knows, the fish chiefly swims, is always single, but may be divided into halves. Most of our common fishes have the halves of the tail-fin of equal size; others, such as the sharks, sturgeons, etc., having the upper half greatly exceeding the lower half of the tail-fin in size. In one species of shark, named the Thresher or Fox-shark, the upper half of the tail-fin appears enormously developed as compared with the lower half; and the names of this species have been derived from the use the fish makes of its tail in lashing the water, and from the long-tailed appearance suggesting a resemblance to the familiar Reynard of the land. In fishes the tail-fin is always placed vertically, or in the same line as the body, and moves from side to side; while in the whales--which are not fishes, but Mammalia or quadrupeds possessing fish-like bodies--the tail-fin is placed across the body. Some fishes may want arms or legs--that is, the pectoral or ventral fins; the eels, for example, possessing no ventral fins. The flying-fishes, on the contrary, possess a very large development of the pectoral or breast fins, and support themselves temporarily in the air by their aid.

Fishes are usually very well provided in the matter of teeth. What would be thought of a quadruped which had teeth not only in its jaws, but had its tongue, its palate, the sides and floor of its mouth, and other parts, also bearing rows of these structures? Yet such is the case with many fishes. Then, also, where the teeth of one set in fishes are lost, or destroyed through the natural wear and tear to which they are subjected, new teeth are developed to supply the place of the lost members. Any one may gain a good idea of the formidable array of teeth in fishes, and of the manner in which one set succeeds another, by inspecting the jaws of a shark in a museum. In fishes the teeth are not implanted in sockets, but are fastened by ligaments to the surface of the bones which bear them. Sometimes one tooth only is developed in fishes. This is the case in the curious, eel-like hag-fishes already mentioned; these fishes possessing but a single large tooth, borne on the palate; and by means of this formidable weapon, which possesses saw-like edges, they bore their way into the bodies of other fishes, and there take up their abode as unwelcome guests. A cod or large haddock may sometimes be found with five or six hags contained in its interior. The parrot-fishes, or Scari, of tropical seas, are so named from their possessing jaws shaped like the beaks of those familiar birds, and these jaws are rendered all the more extraordinary from their being covered or incrusted by numerous small teeth, which are as closely packed on the jaw as paving-stones are in a street, and which serve these fishes as useful instruments when they feed upon the living parts of the hard and limy coral-animals. In the jaws and floor of the mouth of the Port Jackson shark, or in the Eagle rays, or skates, the teeth may be seen to be flat and broad. Such teeth form a regular pavement arranged like a mosaic pattern, and are admirably adapted for crushing whatever substances enter the mouth.

Fishes are well provided in the way of digestive apparatus. A throat or gullet, stomach, intestines, liver, and other glands, serve for the digestion of the food, and a heart and blood-vessels exist for the circulation of the blood thus manufactured from the food. The blood is purified in the gills. Each gill--consisting in common fishes of a supporting “arch” bearing a great number of delicate filaments arranged like the teeth of a comb--may be viewed as simply a network of blood-vessels. The blood, pumped into this network by the heart, is purified by the action of the oxygen gas contained in the pure water which the fish is constantly taking into its gill-chamber by its mouth; while the pure blood is recirculated through the body, and the water used in breathing is got rid of by being ejected behind the “gill-cover” at the neck, so as to allow a fresh inflow to be drawn in by the mouth. The gills of some fishes may be very differently constructed from those of the common members of the class. Thus the lampreys breathe by pouch-like gills which open each by a separate aperture. Seven gill-apertures may be seen on each side of the neck of the common lamprey; and the sharks, skates, and their neighbors also breathe by sac-like gills. Certain curious facts regarding the breathing of fishes will be afterward alluded to. Fishes illustrate plainly what is meant by aquatic or water-breathing. They possess gills or organs, adapted for separating the atmospheric air which is entangled or contained in the water; land animals breathing the same air directly from the atmosphere.

That fishes are wary and active, and possess senses of acute nature, are facts well known to all. The lowest fish, the little clear-bodied lancelet, possesses no brain whatever, and no organ of hearing is developed, while the eyes are at the best of very simple and rudimentary structure. In other fishes, again, the brain and nervous system not only acquire a typical development, but the senses also advance in perfection. The sense of sight is of perfect kind, the eyes of fishes being adapted for seeing in the dense medium in which they live; while the sense of smell is also developed, although, curiously enough, the nostrils, in all except two kinds of fishes--the hag-fishes and the curious Lepidosiren or mud-fish--are pocket-like in nature, and do not open backward, as in higher animals, into the mouth. The sense of taste is not exercised in a high degree by fishes, and it is interesting to observe that the sense of touch appears to reside especially in the sides of the body, on which surfaces a well-marked line--the “lateral line”--may be observed in most fishes. This lateral line is connected with a series of canals or sacs abundantly supplied with nerves. The function of these organs is believed to be that of exercising the sense of touch; and from the manner in which many fishes swim against objects, and bring the sides of their bodies in gentle contact with foreign objects, there would seem to exist strong reasons for supporting the above idea. That fishes “hear” is a well-known fact. No outer ear is developed, but an internal ear--the essential part of the organ of hearing--is found in all fishes except the little lancelet.

While the intelligence or instinct of fishes is not, generally speaking, of a high order, there are not wanting instances to prove that these animals may exhibit traits of character sometimes wanting in higher groups of animals. Any one who has kept gold-fishes must have noted that in time they become more and more familiar with the hand that feeds them, and the experience of aquarium keepers goes to prove that some fishes may even show signs of recognizing friends.

Like the human race, the class of fishes evinces many illustrations of individuals and groups which differ more or less widely from their more commonplace neighbors. To some of the more curious of these “odd fishes” we may next direct attention.

A very singular little group of fishes, for example, is that known to the naturalist by the name Lophobranchii; this term meaning literally “tuft-gilled.” Included in this division are two curious families, of one of which the sea-horses or Hippocampi are the representatives; while to the other family belong their allies, the pipe-fishes. No more interesting forms than these two groups can well be selected from the great class of which they are little-known members. And the interest with which they are regarded by zoologists extends beyond the mere investigation of their outside form or appearance; since they present, in many points of their economy and habits, very marked deviations from what one may call the ordinary course of fish-life.

Imagine a little body from four to six inches in length, topped by a head which in outline exactly resembles that of a horse, and which tapers off below, or rather behind, into a lithe, flexible, and pointed tail, and we may form a rough idea of the general appearance of one of the sea-horses. This little body we shall find to be covered with ganoid plates or scales of hard horny or bony material, exhibiting ridges and angles all over its surface. Two large brilliant eyes, each of which may be moved independently of the other, add to the curious appearance of the head; while to the body itself may be attached long streamers of sea-weed, serving to conceal the little beings as they nestle amid their marine bowers, each looking like some veritable creation of heraldic or mythological kind.

The flexible tail which terminates the body has the important office of mooring or attaching the fishes to any fixed object. As we see them in the aquarium, they are generally poised, as it were, on the tail; the latter being coiled around a bit of sea-weed, while the erect body and head look warily through the waters of their miniature sea. When they detach themselves, they swim about in the erect position by means of the two pectoral or breast fins, which being placed close to the sides of the neck, project like veritable ears, and assist in rendering the equine appearance of the head of still more realistic nature. These fins move with a quick twittering motion, and propel their possessor swiftly through the water; while the back-fin, placed toward the hinder extremity of the body, also assists them in swimming.

Some curious points in the internal structure of the sea-horses warrant a brief notice. As already stated, the gills of an ordinary fish are shaped each like a comb; the teeth of the comb being represented by the delicate processes, each consisting in reality of a network of blood-vessels, in which the blood is exposed to the oxygen of the water, and is thus purified. In the sea-horses, however, the gills do not present this comb-like appearance, but exist in the form of separated tufts or bunches of delicate filaments, which spring from the gill-supports or arches. From this peculiarity the name “tuft-gilled,” already alluded to, is derived, and the pipe-fishes agree in the structure of the gills with the sea-horses. Then, also, as most readers are aware, the gills of ordinary fishes are covered by a horny plate, appropriately named the gill-cover, and it is by sharply compressing the gills with this cover that the water used in breathing is ejected from the gills, so as to make room for a fresh supply. In the sea-horses, however, the gill-cover is not open or free at its under and hinder edges, but is firmly attached all round to the neighboring tissues, and so rendered immovable. At one point in its circumference, however, a small aperture is left, through which the breathing-water escapes from the gills.

The sea-horses are found abundantly in the English Channel, around the coasts of France and Spain, in the Mediterranean Sea, and in the tropical oceans. Several distinct species are known to zoologists, but they closely resemble one another in the essential features just noted. They are lively and intelligent little creatures, and become familiar in time with their possessors. Fixed by their tails, they may be seen actively to dart the head at any passing object adapted for food; while, when they wish to free their bodies from the attached position, they appear to manœuvre with the chin and head in order to effect their purpose. Their food appears to consist of small crustaceans, worms, etc.; and they are known to be especially fond of such delicate titbits as are afforded by the eggs of other fishes.

Perhaps the most curious part of the history of the sea-horses relates to their care of the young. Fishes generally take little or no care of their offspring, and it is therefore the more surprising to encounter in these little beings a singular example of parental fidelity and attachment. Nor, as might be expected, is it the mother-fish who is charged with the task of attending the young. Contrary to the general rule, the male fish assumes the part of nurse, and well and faithfully does he appear to discharge his duties. At the root of the tail in the male sea-horse a curious little pouch is seen. In this pouch the eggs laid by the females--which want the pouch--are deposited, and are therein duly hatched. Nor does the parental duty end here; for after the young are hatched and swim about by themselves, they seek refuge in the pouch during the early or infantile period of their life whenever danger threatens them. This procedure forcibly reminds one of the analogous habits of the kangaroos and their young; but the occurrence is the more remarkable in the lower and presumably less intelligent fish.

Some experiments made on the sea-horses seem to demonstrate the existence of a more than ordinary degree of attachment to the young. Thus when a parent-fish was taken out of the water, the young escaped from the pouch; but on the parent being held over the side of the boat, the young at once swam toward him, and re-entered the pouch without hesitation. Some authorities have not hesitated to express an opinion that the young are nourished within the pouch by some fluid or secretion of its lining membrane. But further observation is certainly necessary before this latter opinion can be relied upon.

The pipe-fishes are very near neighbors of the sea-horses, and derive their name from the thin elongated shape of their bodies, together with the fact that the jaws are prolonged to form a long pipe-like snout, at the extremity of which the mouth opens. These fishes are very lively in all their movements, and dart through the water so quickly that in many cases the eye is unable to follow them. Like the sea-horses, the male pipe-fishes protect and tend their progeny, and exhibit an equal attachment to their young.

These latter features are also well exemplified by the familiar sticklebacks of our ponds and streams. The latter fishes actually build nests for the reception and care of their eggs, the nests being made chiefly or solely by the males; while on the latter, during the process of hatching and in the upbringing of the young, devolves the chief care of protecting and looking after the welfare of the progeny. These instances of the care and duties which devolve on the males, instead of on the mother-parents, appear to reverse the more natural order which almost universally obtains in the case of both lower and higher animals.