Chapter XIII: Introduction: Classification—hydrozoa—eleutheroblastea—milleporina (3)

In many forms two distinct parts of the pneumatophore can be recognised—a distal region lined by chitin,[336] probably representing the sub-umbrellar cavity of the nectocalyx, and a small funnel-shaped region lined by an epithelium, the homology of which is a matter of dispute. It is believed that the gas is secreted by this epithelium. In the Auronectidae the region with secretory epithelium is relatively large and of a more complicated histological character. It is remarkable also that in this family the pore communicates, not with the chitin-lined region, but directly with the epithelium-lined region.

There is no pneumatophore in the Calycophorae, but in this sub-order a diverticulum of an endoderm canal secretes a globule of oil which may serve the same hydrostatic function.

The _stolon_ is the common stem which supports the different zooids of the colony. In the Calycophorae the stolon is a long, delicate, and extremely contractile thread attached at one end to a nectocalyx, and bearing the zooids in discontinuous groups. These groups of zooids arranged at intervals on the stolon are called the "cormidia." The stolon is a tube with very thick walls. Its lumen is lined by a ciliated endoderm with circular muscular processes, and the surface is covered with an ectoderm, also provided with circular muscular processes. Between these two layers there is a relatively thick mesogloea showing on the outer side deep and compound folds and grooves supporting an elaborate system of longitudinal muscular fibres. In many Physonectidae the stolon is long and filamentous, but not so contractile as it is in Calycophorae, but in others it is much reduced in length and relatively stouter. The reduction {302}in length of the stolon is accompanied by a complication of structure, the simple tubular condition being replaced by a spongy complex of tubes covered by a common sheath of ectoderm. In the Auronectidae the stolon is represented by a conical or hemispherical spongy mass bearing the zooids, and in the Rhizophysaliidae and Chondrophoridae it becomes a disc or ribbon-shaped pad spreading over the under side of the pneumatophore.

_Gonozooids._—The gonozooids are simple tubular processes attached to the stolon which bear the Medusae or the degenerate medusiform gonophores. In the Chondrophoridae the gonozooids possess a mouth, but in most Siphonophora they have neither mouth nor tentacles. In some cases, such as _Anthophysa_, the colonies are bisexual—the male and female gonophores being borne by separate gonozooids—but in others (e.g. _Physalia_) the colonies appear to be unisexual.

As a general rule the gonophores of Siphonophora do not escape from the parent colony as free-swimming Medusae, but an exception occurs in _Velella_, which produces a number of small free-swimming Medusae formerly described by Gegenbaur under the generic name _Chrysomitra_. This Medusa has a velum, a single tentacle, eight to sixteen radial canals, and it bears the gonads on the short manubrium. The Medusa of _Velella_ has, in fact, the essential characters of the Anthomedusae.

Our knowledge of the life-history of the Siphonophora is very incomplete, but there are indications, from scattered observations, that in some genera, at least, it may be very complicated.

The fertilised ovum of _Velella_ gives rise to a planula which sinks to the bottom of the sea, and changes into a remarkable larva known as the _Conaria_ larva. This larva was discovered by Woltereck[337] at depths of 600-1000 metres in great numbers. It is very delicate and transparent, but the endoderm is red (the colour so characteristic of animals inhabiting deep water), and it may be regarded as essentially a deep-sea larva. The larva rises to the surface and changes into the form known as the _Ratarula_ larva, which has a simple one-chambered pneumatophore containing a gas, and a rudiment of the sail. In contrast to the _Conaria_, the _Ratarula_ is blue in colour. With the development of the zooids on the under side of this {303}larva (_i.e._ the side opposite to the pneumatophore), a definite octoradial symmetry is shown, there being for some time eight dactylozooids and eight definite folds in the wall of the pneumatophore. This octoradial symmetry, however, is soon lost as the number of folds in the pneumatophore and the number of tentacles increase.

It is probable that in the Siphonophora, as in many other Coelenterata, the production of sexual cells by an individual is no sign that its life-history is completed. There may possibly be two or more phases of life in which sexual maturity is reached.

An example of a complicated life-history is found in the Calycophoran species _Muggiaea kochii_. The embryo gives rise to a form with a single nectocalyx which is like a _Monophyes_, and this by the budding of a second nectocalyx produces a form that has a remarkable resemblance to a _Diphyes_, but the primary nectocalyx degenerates and is cast off, while the secondary one assumes the characters of the single _Muggiaea_ nectocalyx. The stolon of the _Muggiaea_ produces a series of cormidia, and as the sexual cells of the cormidia develop, a special nectocalyx is formed at the base of each one of them, and the group of zooids is detached as an independent colony, formerly known as _Eudoxia eschscholtzii_. In a similar manner the cormidia of _Doramasia picta_ give rise to the sexual free-swimming monogastric forms, known by the name _Ersaea picta_ (Fig. 142). In these cases it seems possible that the production of ripe sexual cells is confined to the _Eudoxia_ and _Ersaea_ stages respectively, but it is probable that in other species the cormidia do not break off from the stolon, or may escape only from the older colonies.

The Siphonophora are essentially free-swimming pelagic {304}organisms. Some of them (Auronectidae) appear to have become adapted to a deep-sea habit, others are usually found in intermediate waters, but the majority occur with the pelagic plankton at or very near the surface of the open sea. Although the order may be said to be cosmopolitan in its distribution, the Siphonophora are only found in great numbers and variety in the sub-tropical and tropical zones. In the temperate and arctic zones they are relatively rare, but _Galeolaria biloba_ and _Physophora borealis_ appear to be true northern forms. The only British species are _Muggiaea atlantica_ and _Cupulita sarsii_. _Velella spirans_ occasionally drifts from the Atlantic on to our western shores, and sometimes great numbers of the pneumatophores of this species may be found cast up on the beach. _Diphyes_ sp., _Physalia_ sp., and _Physophora borealis_ are also occasionally brought to the British shores by the Gulf Stream.

The Calycophorae are usually perfectly colourless and transparent, with the exception of the oil-globule in the oleocyst, which is yellow or orange in colour. Many of the other Siphonophora, however, are of a transparent, deep indigo blue colour, similar to that of many other components of the plankton.

Most of the Siphonophora, although, strictly speaking, surface animals, are habitually submerged; the large pneumatophores of _Velella_ and _Physalia_, however, project above the surface, and these animals are therefore frequently drifted by the prevailing wind into large shoals, or blown ashore. At Mentone, on the Mediterranean, _Velella_ is sometimes drifted into the harbour in countless numbers. Agassiz mentions the lines of deep blue Velellas drifted ashore on the coast of Florida; and a small species of blue _Physalia_ may often be seen in long lines on the shore of some of the islands of the Malay Archipelago.

The food of most of the Siphonophora consists of small Crustacea and other minute organisms, but some of the larger forms are capable of catching and devouring fish. It is stated by Bigelow[338] that a big _Physalia_ will capture and devour a full-grown Mackerel. The manner in which it feeds is described as follows:—"It floats on the sea, quietly waiting for some heedless individual to bump its head against one of the tentacles. The fish, on striking, is stung by the nettle-cells, and fastened probably by them to the tentacle. Trying to run away the fish pulls on the {305}tentacle. The tension on its peduncle thus produced acts as a stimulus on apparently some centre there which causes it to contract. The fish in this way is drawn up so that it touches the sticky mouths of the squirming siphons [_i.e._ gastrozooids]. As soon as the mouths, covered as they are with a gluey substance and provided with nettle-cells, touch the fish they stick fast, a few at first, and gradually more. The mouths open, and their lips are spread out over the fish until they touch, so that by the time he is dead the fish is enclosed in a tight bag composed of the lips of a dozen or more siphon mouths. Here the fish is digested. As it begins to disintegrate partially digested fragments are taken into the stomachs of the attached siphons (gastrozooids). When they have become gorged they detach themselves from the remains of the fish, the process of digestion is completed in the stomachs, and the nutrient fluid is distributed...."

In consequence of the very unsatisfactory state of our knowledge of the life-history of the Siphonophora the classification of the order is a matter of unusual difficulty.

SUB-ORDER I. CALYCOPHORAE.

The character which distinguishes this sub-order is the absence of a pneumatophore.

The colony usually consists of a long, slender, contractile stolon, provided at one end with one, two, or several nectocalyces. Upon the stolon are arranged several groups ("cormidia") of polymorphic zooids.

The nectocalyces have a well-developed velum, four radial canals, and a muscular umbrella-wall. A special peculiarity of the nectocalyx of this sub-order is a diverticulum (oleocyst) from one of the radial canals, containing a coloured globule of oil. The function of this oil-globule is probably similar to that of the pneumatophore, and assists the muscular efforts of the nectocalyces in keeping the colony afloat. One of the nectocalyces of each colony exhibits on one side a deep ectodermic fold, which is frequently converted into a pit. At the bottom of this pit is attached the end of the stolon, the whole of which with its numerous cormidia can be withdrawn into the shelter of the pit when danger threatens. The cormidia consist of at least four {306}kinds of zooids: a gastrozooid with a trumpet-shaped mouth armed with nematocysts, a long dactylozooid provided with a series of tentilla, and a rudimentary gonozooid bearing numbers of male or female medusiform gonophores. These three kinds of zooids are partially covered and protected by a bent shield-shaped phyllozooid or hydrophyllium.

Each of the cormidia is unisexual, but the colony as a whole is usually hermaphrodite, the male and female cormidia regularly alternating, or the male cormidia being arranged on the nectocalycine half and the female cormidia on the opposite half of the stolon.

The families of the Calycophorae are:—

FAM. 1. MONOPHYIDAE.—In this family there is a single conical or mitre-shaped nectocalyx. The cormidia become detached as free-swimming _Eudoxia_ or _Ersaea_ forms.

Sub-Fam. 1. SPHAERONECTINAE.—The primary nectocalyx persists throughout life—_Monophyes_ and _Sphaeronectes_.

Sub-Fam. 2. CYMBONECTINAE.—The primary nectocalyx is thrown off, and is replaced by a secondary and permanent nectocalyx—_Cymbonectes_, _Muggiaea_, and _Doramasia_.

FAM. 2. DIPHYIDAE.—The primary mitre-shaped nectocalyx is thrown off and replaced by two secondary rounded, prismatic, or pyramidal, heteromorphic nectocalyces.

This family contains several sub-families, which are arranged in two groups: the Diphyidae Oppositae, in which the two secondary bells are opposite one another, and do not exhibit pronounced ridges; and the Diphyidae Superpositae, in which one of the two secondary nectocalyces is situated in front of the other, and each nectocalyx is provided externally with very definite and often wing-like ridges. In all the Diphyidae Oppositae the cormidia remain attached, whereas in most of the Diphyidae Superpositae they become free-swimming, as in the Monophyidae.

The sub-families of the DIPHYIDAE OPPOSITAE are:—

Sub-Fam. 1. AMPHICARYONINAE.—One of the two secondary nectocalyces becomes flattened above to form a shield, and at the same time its sub-umbrellar cavity is atrophied, and its radial canals reduced. _Mitrophyes_, Atlantic Ocean.

Sub-Fam. 2. PRAYINAE.—The colony exhibits a pair of large, obtuse nectocalyces, with a relatively small sub-umbrellar cavity. _Praya_, Mediterranean and Atlantic.

{307}Sub-Fam. 3. DESMOPHYINAE.—The colony bears a large number of reserve or tertiary nectocalyces arranged in two rows. _Desmophyes_, Indian Ocean.

Sub-Fam. 4. STEPHANOPHYINAE.—There are four nectocalyces arranged in a horizontal plane. Each one of the cormidia bears a nectocalyx, which is periodically replaced. This sub-family is constituted for _Stephanophyes superba_ from the Canary Islands. It attains a length of 25 cm., and is probably the largest and most beautiful of all the Calycophoridae.[339]

The group DIPHYIDAE SUPERPOSITAE contains the following:—

Sub-Fam. 1. GALEOLARINAE.—_Galeolaria._

Sub-Fam. 2. DIPHYOPSINAE.—_Diphyes._

Sub-Fam. 3. ABYLINAE.—_Abyla._

These sub-families differ from one another in the character and shape of the nectocalyces and in other characters. They have a world-wide distribution, _Diphyes_ and _Galeolaria_ extending north into the Arctic Seas. _Diphyes_ is British.

FAM. 3. POLYPHYIDAE.—The nectocalyces are numerous, and superposed in two rows. The cormidia remain attached.

The family contains the genera _Polyphyes_ and _Hippopodius_, both probably cosmopolitan in warm waters.

SUB-ORDER II. PHYSOPHORAE.

In this sub-order the primary nectocalyx gives rise to a definite pneumatophore. There are four families.

FAM. 1. PHYSONECTIDAE.—In this, the largest family of the sub-order, there is a monothalamic pneumatophore supporting a stolon, which in some forms is of great length, but in others is reduced to a stump or pad, on which there are usually found several nectocalyces, hydrophyllia, gastrozooids, gonozooids, and tentilla.

The principal sub-families are:—

AGALMINAE.—With a long stolon, bearing at the upper end (_i.e._ the end next to the pneumatophore) two rows of nectocalyces. The other zooids are arranged in cormidia on the stolon, each covered by a hydrophyllium. Dactylozooids with tentilla. _Agalma_ and _Cupulita_, Mediterranean Sea.

APOLEMINAE.—Similar to the above, but without tentilla. {308}_Apolemia_—this genus attains a length of two or three metres. Mediterranean Sea. _Dicymba_, Indian Ocean.

PHYSOPHORINAE.—The pneumatophore larger in proportion than it is in the preceding families. The stolon is short, and bears rows of nectocalyces at the upper end. The gastrozooids, dactylozooids, and gonozooids are arranged in verticils on the lower expanded part of the stolon. Hydrophyllia absent. _Physophora_, cosmopolitan in the areas of warm sea water.

FAM. 2. AURONECTIDAE.—The pneumatophore is large. The stolon is reduced to a spongy mass of tissue on the under side of the pneumatophore, and this bears numerous cormidia arranged in a helicoid spiral. Projecting from the base of the pneumatophore there is a peculiar organ called the "aurophore," provided with an apical pore. This organ has been described as a specially modified nectocalyx, but it is probably a specialised development of the epithelium-lined portion of the pneumatophore of other Physophorae. The Auronectidae are found only at considerable depths, 300 to 1400 fathoms, and are probably specially adapted to that habitat. _Rhodalia_, _Stephalia_, Atlantic Ocean.

FAM. 3. RHIZOPHYSALIIDAE.—The pneumatophore is large, or very large, in this family. The zooids are arranged in horizontal rows on the under side of the pneumatophore (_Physalia_), or in a helicoid spiral on a short stolon (_Epibulia_). There are no nectocalyces nor hydrophyllia.

The genus _Physalia_ is the notorious "Portuguese Man-of-War." The pneumatophore is a large bladder-like vesicle, sometimes attaining a length of 12 cm. One species described by Haeckel under the generic name _Caravella_ has a pneumatophore 30 cm. and more in length, and dactylozooids attaining a length of 20 metres. It is a curious fact that only the male colonies of _Physalia_ are known, and it is suggested that the female may have quite a different form.[340] _Epibulia_ has a much smaller bladder than _Physalia_. Both genera have a cosmopolitan distribution at the surface of the warm seas.

FAM. 4. CHONDROPHORIDAE.—This family stands quite by itself in the sub-order Physophorae, and is placed in a separate division of the sub-order by Chun, who gives it the name TRACHEOPHYSA. The essential distinguishing characters of the family are {309}the large polythalamic pneumatophore and the single large central gastrozooid.

The colony is disc-shaped, and has a superficial resemblance to a Medusa. On the upper side is the flattened pneumatophore, covered by a fold of tissue continuous with that at the edge of the disc. In _Velella_ a vertical triangular sail or crest rises from the upper side, but this is absent in _Porpita_.

The mouth of the gastrozooid opens into a large digestive cavity, and between this and the under surface of the pneumatophore there is a glandular spongy tissue called the liver. The liver extends over the whole of the under side of the pneumatophore, and sends processes round the edge of the disc into the tissues of its upper surface. Intimately associated with the liver, and penetrating its interstices, is an organ which appears to be entirely composed of nematocysts, derived from the ectoderm, and called the central organ. At the margin of the disc there is a fringe of simple digitiform dactylozooids, and between the dactylozooids and the centrally placed gastrozooid are numerous gonozooids. Each of the gonozooids is provided with a distinct mouth, and bears the gonophores, which escape before the ripening of the gonads as the free-swimming Medusae called _Chrysomitra_. The pneumatophore consists of a number of annular chambers arranged in a concentric manner round the central original chamber formed from a modified zooid. These annular chambers are in communication with one another, and have each two pores (pneumatopyles) opening above to the exterior. The most remarkable feature, however, of the system is a series of fine branching tubes ("tracheae"), which pass from the annular chambers of the pneumatophore downwards into the hepatic mass and ramify there.

There are two well-known genera: _Velella_ with a sail, and _Porpita_ without a sail. They are both found at the surface of the warmer regions of the great oceans and in the Mediterranean. _Velella_ sometimes drifts on to British coasts from the Atlantic.

The genus _Discalia_ has a much more simple octoradial structure. It was found at depths of 2600 and 2750 fathoms in the Pacific Ocean.

{310}CHAPTER XII

COELENTERATA (_CONTINUED_): SCYPHOZOA = SCYPHOMEDUSAE

CLASS II. SCYPHOZOA = SCYPHOMEDUSAE

The Scyphozoa are jelly-fishes, usually found floating at or near the surface of the sea. A few forms (Stauromedusae) are attached to rocks and weeds by a stalked prolongation of the aboral region of the umbrella. With this exception, however, they are all, in the adult stage, of the Medusa type of structure, having a bell-shaped or discoid umbrella, from the under surface of which depends a manubrium bearing the mouth or (in Rhizostomata) the numerous mouths.

Although many of the species do not exceed an inch or a few inches in diameter, others attain a very great size, and it is among the Scyphozoa that we find the largest individual zooids of the Coelenterata. Some Discophora have a disc three or four feet in diameter, and one specimen obtained by the Antarctic Expedition of 1898-1900 weighed 90 lbs.[341] The common jelly-fish, _Aurelia_, of our coasts belongs to a species that appears to be very variable in general characters as well as in size. Specimens obtained by the "Siboga" in the Malay Archipelago ranged from 6 to 64 cm. in diameter. The colour is very variable, shades of green, blue, brown, and purple being conspicuous in many species; but a pale milky-blue tint is perhaps the most prevalent, the tissues being generally less transparent than they are in the Medusae of the Hydrozoa. The colour of the Cubomedusae is usually yellow or brown, but _Charybdea xaymacana_ is colourless and transparent. The deep-sea species, particularly the Periphyllidae, have usually an opaque brown or dark red colour. The surface-swimming {311}forms, such as the common _Aurelia_, _Pelagia_, _Cyanaea_, are usually of a uniform pale milky-blue or green colour. Generally the colour is uniformly distributed, but sometimes the surface of the umbrella is freckled with irregular brown or yellow patches, as in _Dactylometra_ and many others. There is frequently a special colour in the statorhabs which renders them conspicuous in the living jelly-fish, and the lips, or parts of the lips, of the manubrium have usually a different colour or tone to that of the umbrella.

There is no reason to believe that the general colour of any of these jelly-fishes has either a protective or a warning significance. Nearly all the larger species, whether blue, green, or brown in colour, can be easily seen from a considerable distance, and the colours are not sufficiently bright or alarming to support the belief that they can serve the purpose of warning either fish or birds of the presence of a dangerous stinging animal. It is possible, however, that the brighter spots of colour that are often noticed on the tips of the tentacles and on the lips may act as a lure or bait in attracting small fish and Crustacea.

Some of the Scyphozoa are phosphorescent, but it is a singular fact that there are very few recorded observations concerning the phosphorescence or the absence of it in most of the species. The pale blue light of _Pelagia noctiluca_ or _P. phosphora_ can be recognised from the deck of a ship in the open ocean, and they are often the most brilliant and conspicuous of the phosphorescent organisms.

The food of the Scyphozoa varies a good deal. _Charybdea_ and _Periphylla_, and probably many others with large mouths, will capture and ingest relatively large fish and Crustacea; but _Chrysaora isosceles_[342] apparently makes no attempt to capture either Copepoda or small fish, but preys voraciously upon Anthomedusae, Leptomedusae, Siphonophora, Ctenophora, and pelagic worms. Very little is known about the food of the Rhizostomata, but the small size of the mouths of these forms suggests that their food must also be of minute size. The frequent association of small fish with the larger jelly-fish is a matter of some interest that requires further investigation. In the North Sea young whiting are the constant guests of _Cyanaea capillata_.[343] Over a {312}hundred young horse-mackerel (_Caranx trachurus_) may be found sheltering under the umbrella of _Rhizostoma pulmo_. As the animal floats through the water the little fishes hover round the margin, but on the slightest alarm dart into the sub-umbrella cavity, and ultimately seek shelter in the sub-genital pits.[344]

Two species of fish accompany the American Medusa _Dactylometra lactea_, one a Clupeoid, the other the young of the Butter-fish (_Stromateus triacanthus_). According to Agassiz and Mayer[345] this is not an ordinary case of mutualism, as the fish will tear off and devour fragments of the tentacles and fringe of the Medusa, whilst the Medusa will in its turn occasionally capture and devour one of the fish.

A great many of the Scyphozoa, particularly the larger kinds, have the reputation of being able to sting the human skin, and in consequence the name Acalephae[346] was formerly used to designate the order. Of the British species _Aurelia aurita_ is almost harmless, and so is the rarer _Rhizostoma pulmo_; but the nematocysts on the tentacles of _Cyanaea_, _Chrysaora_, and _Pelagia_ can inflict stings on the more delicate parts of the skin which are very painful for several hours, although the pain has been undoubtedly greatly exaggerated in many popular works.

The soft structure of the Medusae does not favour their preservation in the rocks, but the impressions left by several genera, all belonging apparently to the Rhizostomata, have been found in Cambrian, Liassic, and Cretaceous deposits.

There is reason to believe that many Scyphozoa exhibit a considerable range of variation in the symmetry of the most important organs of the body. Very little information is, however, at hand concerning the variation of any species except _Aurelia aurita_, which has been the subject of several investigations. Browne[347] has found that in a local race of this species about 20 per cent exhibit variations from the normal in the number of the statorhabs, and about 2 per cent in the number of gastric pouches.

The Scyphozoa are not usually regarded as of any commercial or other value, but in China and Japan two species of Rhizostomata (_Rhopilema esculenta_ and _R. verrucosa_) are used as food. {313}The jelly-fish is preserved with a mixture of alum and salt or between the steamed leaves of a kind of oak. To prepare the preserved food for the table it is soaked in water, cut into small pieces, and flavoured. It is also stated that these Medusae are used by fishermen as bait for file-fish and sea-bream.[348]

In general structure the Scyphozoa occupy an intermediate position between the Hydrozoa and the Anthozoa. The very striking resemblance of the body-form to the Medusa of the Hydrozoa, and the discovery of a fixed hydriform stage in the life-history of some species, led the older zoologists to the conclusion that they should be included in the class Hydrozoa. Recently the finer details of development have been invoked to support the view that they are Anthozoa specially adapted for a free-swimming existence, but the evidence for this does not appear to us to be conclusive.

They differ from the Hydrozoa and resemble the Anthozoa in the character that the sexual cells are matured in the endoderm, and escape to the exterior by way of the coelenteric cavity, and not directly to the exterior by the rupture of the ectoderm as in all Hydrozoa. They differ, on the other hand, from the Anthozoa in the absence of a stomodaeum and of mesenteries.

The view that the Scyphozoa are Anthozoa is based on the belief that the manubrium of the former is lined by ectoderm, and is homologous with the stomodaeum of the latter; and that the folds of mesogloea between the gastric pouches are homologous with the septa.[349]

The Scyphozoa, notwithstanding their general resemblance to the Medusae of Hydrozoa, can be readily distinguished from them by several important characters. The absence of a velum in all of them (except the Cubomedusae) is an important and conspicuous character which gave to the class the name of Acraspeda. The velum of the Cubomedusae can, however, be distinguished from that of the Craspedote Medusae (_i.e._ the Medusae of the Hydrozoa) by the fact that it contains endodermal canals.

Sense-organs are present in all Scyphozoa except some of the Stauromedusae, and they are in the form of statorhabs (tentaculocysts), bearing statoliths at the extremity, and in many species, {314}at the base or between the base and the extremity, one or more eyes. These organs differ from the statorhabs of the Hydrozoa in having, usually, a cavity in the axial endoderm; but as they are undoubtedly specially modified marginal tentacles, they are strictly homologous in the two classes. In nearly all the Scyphozoa these organs are protected by a hood or fold formed from the free margin of the umbrella, and this character, although not of great morphological importance, serves to distinguish the common species from the Craspedote Medusae. It was owing to this character that Forbes gave the name STEGANOPHTHALMATA, or "covered-eyed Medusae," to the class.

Another character of some importance is the presence in the coelenteric cavity of all Scyphozoa of clusters or rows of delicate filaments called the "phacellae." These filaments are covered with a glandular epithelium, and are usually provided with numerous nematocysts. They have a considerable resemblance to the acontia of certain Anthozoa, and are probably mainly digestive in function. These three characters, in addition to the very important character of the position and method of discharge of the sexual cells already referred to, justify the separation of the Scyphozoa from the Medusae of the Hydrozoa as a distinct class of Coelenterata.

The umbrella of the Scyphozoa varies a good deal in shape. It is usually flattened and disc-like (Discophora), but it may be almost globular (_Atorella_), conical (some species of _Periphylla_), or cubical (Cubomedusae). It is divided into an aboral and a marginal region by a circular groove in the Coronata. The margin may be almost entire, marked only by notches where the statorhabs occur, or deeply lobed as in the Coronata and many Discophora. Marginal tentacles are present in all but the Rhizostomata, and may be few in number, four in _Charybdea_, eight in _Ulmaris_ (Fig. 143), or very numerous in _Aurelia_ and many others. The tentacles may be short (_Aurelia_), or very long as in _Chrysaora isosceles_, in which they extend for a length of twenty yards from the disc.

The manubrium of the Scyphozoa is usually quadrangular in section, and in those forms in which the shape is modified in the adult Medusa the quadrangular shape can be recognised in the earlier stages of development. The four angles of the manubrium are of importance in descriptive anatomy, as the planes drawn {315}through the angles to the centre of the manubrium are called "perradial," while those bisecting the perradial planes and passing therefore through the middle line of the flat sides of the manubrium are called "interradial."

The free extremity of the manubrium in many Scyphozoa is provided with four triangular perradial lips, which may be simple or may become bifurcated or branched, and have frequently very elaborate crenate edges beset with batteries of nematocysts. In _Pelagia_ and _Chrysaora_ and other genera these lips hang down from the manubrium as long, ribbon-like, folded bands, and according to the size of the specimen may be a foot or more in length, or twice the diameter of the disc.

In the Rhizostomata a peculiar modification of structure takes place in the fusion of the free edges of the lips to form a suture perforated by a row of small apertures, so that the lips have the appearance of long cylindrical rods or tubes attached to the manubrium, and then frequently called the "oral arms." The oral arms may be further provided with tentacles of varying size and importance. In many Rhizostomata branched or knobbed processes project from the outer side of the upper part of the oral arms. These are called the "epaulettes."

The lumen of the manubrium leads into a large cavity in the disc, which is usually called the gastric cavity, and this is extended into four or more interradial or perradial gastric pouches. The number of these pouches is usually four, but in this, as in {316}other features of their radial symmetry, the jelly-fish frequently exhibit duplication or irregular variation of the radii.[350]

The gastric pouches may extend to the margin of the disc, where they are united to form a large ring sinus, or they may be in communication at the periphery by only a very narrow passage (Cubomedusae). In the Discophora the gastric pouches, however, do not extend more than half-way to the margin, and they may be connected with the marginal ring-canal by a series of branched interradial canals. Between the gastric pouches in these forms branched perradial canals pass from the gastric cavity to the marginal ring canal, and the system of canals is completed by unbranched "adradial" canals passing between the perradials and interradials from the sides of the gastric pouches to the ring-canal (Fig. 143).

In the Discophora there are four shallow interradial pits or pouches lined by ectoderm on the under side of the umbrella-wall. As these pits correspond with the position of the gonads in the gastric pouches they are frequently called the "sub-genital pits." In the Stauromedusae and Cubomedusae they are continued through the interradial gastric septa to the aboral side of the disc, and they are generally known in these cases by the name "interradial funnels." The functions and homologies of these ectodermic pits and funnels are still uncertain.

The Scyphozoa are usually dioecious, but _Chrysaora_ and _Linerges_ are sometimes hermaphrodite. The female Medusae can usually be distinguished from the male by the darker or brighter colour of the gonads, which are band-shaped, horseshoe-shaped, or circular organs, situated on the endoderm of the interradial gastric pouches. They are, when nearly ripe, conspicuous and brightly coloured organs, and in nearly all species can be clearly seen through the transparent or semi-transparent tissues of the disc. The reproductive cells are discharged into the gastric cavity and escape by the mouth. The eggs are probably fertilised in the water, and may be retained in special pouches on the lips of the manubrium until the segmentation is completed.[351] Asexual reproduction does not occur in the free-swimming or adult stage of any Scyphozoa. In some cases (probably exceptional) the development is direct. In _Pelagia_, for example, it is known that the fertilised egg gives {317}rise to a free-swimming Medusa similar in all essential features to the parent.

In many species, however, the planula larva sinks to the bottom of the sea, develops tentacles, and becomes attached by its aboral extremity to a rock or weed, forming a sedentary asexual stage of development with a superficial resemblance to a _Hydra_. This stage is the "Scyphistoma," and notwithstanding its simple external features it is already in all essential anatomical characters a Scyphozoon.

The Scyphistoma may remain as such for some time, during which it reproduces by budding, and in some localities it may be found in great numbers on seaweeds and stones.[352]

In the course of time, however, the Scyphistoma exhibits a ring-like constriction of the body just below the crown of tentacles, and as this deepens the general features of a Scyphomedusa are developed in the free part above the constriction. In time this free part escapes as a small free-swimming jelly-fish, called an "Ephyra," while the attached part remains to repeat the process. In many species the first constriction is followed by a second immediately below it, then a third, a fourth, and so on, until the Scyphistoma is transformed into a long series of narrow discs, each one acquiring, as it grows, the Ephyra characters. Such a stage has been compared in form to a pile of saucers, and is known as the "Strobila."

The Ephyra differs from the adult in many respects. The disc is thin and flat, the manubrium short, the margin of the umbrella deeply grooved, while the statorhabs are mounted on bifid lobes which project outwards from the margin. The stabilisation of the Scyphistoma is a process of reproduction by transverse fission, and in some cases this is supplemented by gemmation, the Scyphistoma giving rise to a number of buds which become detached from the parent and subsequently undergo the process of strobilisation.

The Scyphistoma of _Nausithoe_ presents us with the most {318}remarkable example of this mode of reproduction (Fig. 144), as it forms an elaborate branching colony in the substance of certain species of sponges. The ectoderm secretes a chitinous perisarc, similar to that of the hydrosome stage of many of the Hydrozoa, and consequently _Stephanoscyphus_ (_Spongicola_), as this Scyphistoma was called, was formerly placed among the Gymnoblastea. It is remarkable that, although the Scyphozoan characters of _Spongicola_ were proved by Schulze[353] in 1877, a similar Scyphistoma stage has not been discovered in any other genus.

ORDER I. CUBOMEDUSAE.

Scyphozoa provided with four perradial statorhabs, each of which bears a statolith and one or several eyes. There are four interradial tentacles or groups of tentacles. The stomach is a large cavity bearing four tufts of phacellae (Fig. 145, _Ph_), situated interradially. There are four flattened perradial gastric pouches in the wall of the umbrella which communicate with the stomach by the gastric ostia (_Go_). These pouches are separated from one another by four interradial septa; and the long leaf-like gonads are attached by one edge to each side of the septa. In many respects the Cubomedusae appear to be of simple structure, but the remarkable differentiation of the eyes and the occurrence of a velum (p. 313) suggest that the order is a highly specialised offshoot from a primitive stock.

FAM. 1. CHARYBDEIDAE.—Cubomedusae with four interradial tentacles.

{319}_Charybdea_ appears to have a very wide geographical distribution. Some of the species are usually found in deep water and come to the surface only occasionally, but others (_C. xaymacana_) are only found at the surface of shallow water near the shore. The genus can be easily recognised by the four-sided prismatic shape of the bell and the oral flattened expansion of the base of the tentacles. The bell varies from 2-6 cm. in length (or height) in _C. marsupialis_, but a giant form, _C. grandis_,[354] has recently been discovered off Paumotu Island which is as much as 23 cm. in height. The colour is usually yellow or brown, but _C. grandis_ is white and _C. xaymacana_ perfectly transparent.

"_Charybdea_ is a strong and active swimmer, and presents a very beautiful appearance in its movements through the water; the quick, vigorous pulsations contrasting sharply with the sluggish contractions seen in most Scyphomedusae." It appears to be a voracious feeder. "Some of the specimens taken contained in the stomach small fish, so disproportionately large in comparison with the stomach that they lay coiled up, head overlapping tail."[355]

Very little is known of the development, but it is possible that _Tamoya punctata_, which lacks gonads, phacellae, and canals in the velum, may be a young form of a species of _Charybdea_.

FAM. 2. CHIRODROPIDAE.—Cubomedusae with four interradial groups of tentacles.

This family is represented by the genera _Chirodropus_ from the Atlantic and _Chiropsalmus_ from the Indian Ocean and the coast of North Carolina.

FAM. 3. TRIPEDALIIDAE.—Cubomedusae with four interradial groups of three tentacles.

The single genus and species _Tripedalia cystophora_ has only been found in shallow water off the coast of Jamaica. Specimens of this species were kept for some time by Conant in an aquarium, and produced a number of free-swimming planulae which settled on the glass, and quickly developed into small hydras with a mouth and four tentacles. The further development of this sedentary stage is unfortunately not known.

{320}ORDER II. STAUROMEDUSAE.

This order contains several genera provided with an aboral stalk which usually terminates in a sucker, by means of which the animal is temporarily fixed to some foreign object. There can be little doubt that this sedentary habit is recently acquired, and the wide range of the characteristic features of the order may be accounted for as a series of adaptations to the change from a free-swimming to a sedentary habit.

It is difficult to give in a few words the characters of the order, but the Stauromedusae differ from other Scyphozoa in the absence or profound modification in structure and function of the statorhabs. They are absent in _Lucernaria_ and the Depastridae, and very variable in number in _Haliclystus_.

The statorhab of _Haliclystus_ terminates in a spherical knob, which is succeeded by a large annular pad or collar bearing a number of glandular cells which secrete a sticky fluid. At the base of the organ there is a rudimentary ocellus. The number is very variable, and sometimes they are abnormal in character, being "crowned with tentacles." There can be little doubt that the principal function of these organs is not sensory but adhesive, and hence they have received the names "colletocystophores" and "marginal anchors," but they are undoubtedly homologous with the statorhabs of other Scyphozoa.

The tentacles are short and numerous, and are frequently mounted in groups on the summit of digitate outgrowths from the margin of the umbrella. They are capitate, except in _Tessera_, the terminal swelling containing a battery of nematocysts.

Very little is known concerning the life-history and development of the Stauromedusae.

FAM. 1. LUCERNARIIDAE.—Marginal lobes digitate, bearing the capitate tentacles in groups. _Haliclystus auricula_ is a common form on the shores of the Channel Islands, at Plymouth, and other localities on the British coast. It may be recognised by the prominent statorhabs situated in the bays between the digitate lobes of the margin of the umbrella. Each of the marginal lobes bears from 15 to 20 capitate tentacles. It is from 2 to 3 cm. in length. The genus occurs in shallow water {321}off the coasts of Europe and North America, extending south into the Antarctic region.

_Lucernaria_ differs from _Haliclystus_ in the absence of statorhabs. It has the same habit as _Haliclystus_, and is often found associated with it. _L. campanulata_ is British.

_Halicyathus_ is similar in external features to _Haliclystus_, but differs from it in certain important characters of the coelenteric cavities. It is found off the coasts of Norway, Greenland, and the Atlantic side of North America.

In _Capria_, from the Mediterranean, the tentacles are replaced by a denticulated membrane bearing nematocysts.

The rare genus _Tessera_, from the Antarctic Ocean, differs from all the other Stauromedusae in having no stalk and in having only a few relatively long non-capitate tentacles. If _Tessera_ is really an adult form it should be placed in a separate family, but, notwithstanding the presence of gonads, it may prove to be but a free-swimming stage in the history of a normally stalked genus.

FAM. 2. DEPASTRIDAE.—The margin of the umbrella is provided with eight shallow lobes bearing one or more rows of tentacles. Statorhabs absent.

_Depastrum cyathiforme_ occurs in shallow water at Plymouth, Port Erin, and in other localities on the coasts of Britain and Norway. The tentacles are arranged in several rows on the margin of the umbrella. In _Depastrella_ from the Canaries there is only one row of marginal tentacles.

FAM. 3. STENOSCYPHIDAE.[356]—Stauromedusae with simple undivided umbrella margin. The eight principal tentacles are converted into adhesive anchors. Secondary tentacles arranged in eight adradial groups. _Stenoscyphus inabai_, 25 cm., Japan.

ORDER III. CORONATA.[357]

The external surface of the umbrella is divided into two regions, an aboral region and a marginal region, by a well-marked circular groove (the coronal groove). The aboral region is usually smooth and undivided, but it is an elongated dome, {322}thimble- or cone-shaped, in marked contrast to the flattened umbrella of the Discophora. The margin is divided into a number of triangular or rounded lobes, and these are continued as far as the coronal groove as distinct areas delimited by shallow grooves on the surface of the umbrella. The tentacles arise from the grooves between the marginal areas, and are provided with expanded bases called the pedalia. The manubrium may be short or moderately long, but it is never provided with long lips.

FAM. 1. PERIPHYLLIDAE.[358]—Coronata with four or six statorhabs.

In _Pericolpa_ (Kerguelen) there are only four tentacles and four statorhabs. In _Periphylla_, a remarkable deep-sea genus from 700 to 2000 fathoms in all seas, but occasionally found at the surface, there are twelve tentacles and four statorhabs. The specimens from deep water have a characteristic dark red-brown or violet-brown colour. They are usually small Medusae, but the umbrella of _P. regina_ is over 21 cm. in diameter. _Atorella_ has six tentacles and six statorhabs.

FAM. 2. EPHYROPSIDAE.—Coronata with eight or more than eight statorhabs.

_Nausithoe punctata_ is a small, transparent jelly-fish, not exceeding 10 mm. in diameter, of world-wide distribution. Its Scyphistoma stage is described on p. 317. _N. rubra_, a species of a reddish colour found at a considerable depth in the South Atlantic and Indian Oceans, is probably an abysmal form. _Palephyra_ differs from _Nausithoe_ in having elongated instead of rounded gonads. _Linantha_ and _Linuche_ differ from the others in having subdivided marginal lobes.

FAM. 3. ATOLLIDAE.—_Atolla_ is a deep-sea jelly-fish of very wide geographical distribution. It is characterised by the multiplication of the marginal appendages, but the number is very irregular. There may be double or quadruple the usual number of marginal lobes, or an indefinite number. There may be sixteen to thirty-two statorhabs, and the number of tentacles is quite irregular. Some of the species attain a considerable size, the diameter of the umbrella of _A. gigantea_ being 150 mm., of _A. valdiviae_ sometimes 130 mm., and of _A. bairdi_ 110 mm.

{323}ORDER IV. DISCOPHORA.

This order contains not only by far the greater number of the species of Scyphozoa, but those of the largest size, and all those that are familiar to the seaside visitor and the mariner under the general term jelly-fish.

They may be distinguished from the other Scyphozoa by several well-marked characters. The umbrella is flattened and disc-shaped or slightly domed, but not divided by a coronary groove. The perradial angles of the mouth are prolonged into long lips, which may remain free (Semaeostomata) or fuse to form an elaborate proboscis (Rhizostomata).

SUB-ORDER I. SEMAEOSTOMATA.

In this sub-order the mouth is a large aperture leading into the cavity of the manubrium, and is guarded by four long grooved and often tuberculated lips. The margin of the umbrella is provided with long tentacles.

FAM. 1. PELAGIIDAE.—Semaeostomata with wide gastric pouches, which are not united by a marginal ring sinus. _Pelagia_, which forms the type of this family, has eight long marginal tentacles. It develops directly from the egg, the fixed Scyphistoma stage being eliminated.[359] It is probably in consequence of this peculiarity of its development and independence of a shore for fixation that _Pelagia_ has become a common and widespread inhabitant of the high seas. In the Atlantic and Indian Oceans _P. phosphora_ occurs in swarms or in long narrow lines many miles in length. It is remarkable for its power of emitting phosphorescent light. In the Atlantic it extends from 50° N. to 40° S., but is rare or absent from the colder regions. _P. perla_ is found occasionally on the west coast of Ireland. _Chrysaora_ differs from _Pelagia_ in the larger number of tentacles. There are, in all, 24 tentacles and 8 statorhabs, separated by 32 lobes of the margin of the umbrella. _C. isosceles_ is occasionally found off the British coast. It passes through a typical Scyphistoma stage in development. _Dactylometra_, a very {324}common jelly-fish of the American Atlantic shores, differs from _Chrysaora_ in having sixteen additional but small tentacles arranged in pairs at the sides of the statorhabs.

FAM. 2. CYANAEIDAE.—Semaeostomata with eight radial and eight adradial pouches, which give off ramifying canals to the margin of the umbrella; but these canals are not united by a ring-canal. The tentacles are arranged in bundles on the margin of the deeply lobed umbrella.

The yellow _Cyanaea capillata_ and the blue _C. lamarcki_ are commonly found on the British coasts.

FAM. 3. ULMARIDAE.—The gastric pouches are relatively small, and communicate with a marginal ring-canal by branching perradial and interradial canals and unbranched adradial canals.

In _Ulmaris prototypus_ (Fig. 143, p. 315) there are only eight long adradial tentacles, and the lips of the manubrium are relatively short. It is found in the South Atlantic.

_Aurelia_ is a well-known and cosmopolitan genus, which may be recognised by the eight shallow lobes of the umbrella-margin beset with a fringe of numerous small tentacles.

SUB-ORDER II. RHIZOSTOMATA.

In this sub-order the lips are very much exaggerated in size, and are fused together by their margin in such a manner that the mouth of the animal is reduced to a number of small apertures situated along the lines of suture. Tentacles are absent on the margin of the umbrella. This sub-order contains some of the largest known jelly-fishes, and exhibits a considerable range of structure. The families are arranged by Maas[360] in three groups.

Group I. ARCADOMYARIA.—Musculature of the disc arranged in feather-like arcades. Oral arms pinnate.

FAM. CASSIOPEIDAE.—There are no epaulettes on the arms. Labial tentacles present. _Cassiopea_ is common in the Indo-Pacific seas, and extends into the Red Sea. It includes a great many species varying in size from 4 to about 12 cm. in diameter.

Group II. RADIOMYARIA.—Musculature arranged in radial tracts. Oral arms bifid.

FAM. CEPHEIDAE.—The genera included in this family differ {325}from the Cassiopeidae in the characters of the group. _Cephea_ is found in the Indo-Pacific Oceans and Red Sea. _Cotylorhiza_ is common in the Mediterranean Sea and extends into the Atlantic Ocean.

Group III. CYCLOMYARIA.—The group contains the majority of the Rhizostomata. Musculature arranged in circular bands round the disc. Oral arms primarily trifid, but becoming in some cases very complicated. The principal families are:—

FAM. RHIZOSTOMATIDAE.—With well-marked epaulettes, and sixteen radial canals passing to the margin of the umbrella.

_Rhizostoma pulmo_ (= _Pilema octopus_), a widely distributed species, is often found floating at the surface off the western coasts of Scotland and Ireland, and sometimes drifts up the English Channel into the German Ocean in the autumn. The umbrella is about two feet in diameter, and the combined length of the umbrella and arms is four feet. The colour varies considerably, but that of a specimen obtained off Valencia in 1895 was described as follows: "The colour of the umbrella was pale green, with a deep reddish margin. Arms bright blue."[361]

The family includes _Stomolophus_, of the Pacific and Atlantic coasts of America, in which the oral arms are united at the base, and _Rhopilema_, the edible Medusa of Japan and China.

FAM. LYCHNORHIZIDAE.—Here there are only eight radial canals reaching as far as the margin of the umbrella, and eight terminating in the ring-canal. There are no epaulettes, and the oral tentacles are often very long. The family includes _Lychnorhiza_ from the coast of Brazil, _Crambione_ from the Malay Archipelago, and _Crambessa_ from the Atlantic shores of France and Spain and from Brazil and Australia. The last-named genus has been found in brackish water at the mouth of the Loire.

In the families LEPTOBRACHIIDAE and CATOSTYLIDAE there are eight radial canals reaching the margin of the umbrella, and between them a network of canals with many openings into the ring-canal. In a few of the Leptobrachiidae the intermediate canal-network has only eight openings into the ring-canal, as in the Lychnorhizidae.

{326}CHAPTER XIII

COELENTERATA (_CONTINUED_): ANTHOZOA = ACTINOZOA—GENERAL CHARACTERS—ALCYONARIA

CLASS III. ANTHOZOA = ACTINOZOA

Among the familiar objects included in this class are the Sea-anemones, the Stony Corals (Madrepores), the Flexible Corals, the Precious Coral, and the Sea-pens. With the exception of a few species of Sea-anemone, Anthozoa are not commonly found on British sea-shores; but in those parts of the tropical world where coral reefs occur, the shore at low tide is carpeted with various forms of this class, and the sands and beaches are almost entirely composed of their broken-down skeletons.

The majority of the Anthozoa are colonial in habit, a large number of individuals, or zooids as they are called, being organically connected together by a network of nutritive canals, and forming a communal gelatinous or stony matrix for their protection and support. Whilst the individuals are usually small or minute, the colonial masses they form are frequently large. Single colonies of the stony corals form blocks of stone which are sometimes five feet in diameter, and reach a height of two or three feet from the ground. From the tree or shrub-like form assumed by many of the colonies they were formerly included in a class Zoophyta or animal-plants.

But whether the individual polyps are large or small, whether they form colonies in the adult condition or remain independent, they exhibit certain characters in common which distinguish them not only from the other Coelenterata, but from all other animals. When an individual zooid is examined in the living and fully expanded condition, it is seen to possess a cylindrical {327}body, attached at one end (the aboral end) to the common colonial matrix or to some foreign object. At the opposite or free extremity it is provided with a mouth surrounded by a crown of tentacles. In these respects, however, they resemble in a general way some of the Hydrozoa. It is only when the internal anatomy is examined that we find the characters which are absolutely diagnostic of the group.

In the Hydrozoa the mouth leads directly into the coelenteric cavity; in the Anthozoa, however, the mouth leads into a short tube or throat, called the "stomodaeum," which opens into the coelenteric cavity. Moreover, this tube is connected with the body-wall, and is supported by a series of fleshy vertical bands called the mesenteries (Fig. 146). The mesenteries not only support the stomodaeum, but extend some distance below it. Where the mesenteries are free from the stomodaeum their edges are thickened to form the important digestive organs known as the mesenteric filaments (_mf_). It is in the possession of a stomodaeum, mesenteries, and mesenteric filaments that the Anthozoa differ from all the other Coelenterata. There is one character that the Anthozoa share with the Scyphozoa, and that is, that the gonads or sexual cells (G) are derived from the endoderm. They are discharged first into the coelenteric cavity, and then by way of the mouth to the exterior. In the Anthozoa the gonads are situated on the mesenteries.

Nearly all the Anthozoa are sedentary in habit. They begin life as ciliated free-swimming larvae, and then, in a few hours or days, they become attached to some rock or shell at the bottom and immediately (if colonial) start the process of budding, which gives rise to the colonies of the adult stage. Many of the Sea-anemones, however, move considerable distances by gliding {328}over the rocks or seaweeds, others habitually burrow in the sand (_Edwardsia_, _Cerianthus_), and one family (the Minyadidae) are supported by a gas bladder, and float at the surface of the sea. The Sea-pens, too, although usually partly buried in the sand or mud, are capable of shifting their position by alternate distension and contraction of the stalk.[362] The Anthozoa are exclusively marine. With the exception of a few Sea-anemones that are found in brackish or almost fresh water in river estuaries, they only occur in salt sea water. The presence of a considerable admixture of fresh water, such as we find at the mouths of rivers, seems to interfere very materially with the development and growth of all the reef-forming Corals, as will be noticed again in the chapter on coral reefs. A few genera descend into the greatest depths of the ocean, but the home of the Anthozoa is pre-eminently the shallow seas, and they are usually found in great abundance in depths of 0-40 fathoms from the shores of the Arctic and Antarctic lands to the equatorial belt.