Chapter IX

PORIFERA (_CONTINUED_): REPRODUCTION, SEXUAL AND ASEXUAL—PHYSIOLOGY—DISTRIBUTION—FLINTS

The reproductive processes of Sponges are of such great importance in leading us to a true conception of the nature of a sponge that we propose to treat them here in a special section. Both sexual and asexual methods are common; the multiplication of oscula we do not regard as an act of reproduction (p. 174).

A cursory glance at a collection of sponge LARVAE from different groups would suggest the conclusion that they are divisible into two wholly distinct types. One of these is the _amphiblastula_, and the other the _parenchymula_. This was the conclusion accepted by zoologists not long ago. We are indebted to Delage, Maas, and Minchin for dispelling it, and showing that {227}these types are but the extreme terms of a continuous series of forms which have all the same essential constitution and undergo the same metamorphosis.

The amphiblastula of _Sycon raphanus_ (Fig. 111) consists of an anterior half, formed of slender flagellated cells, and a posterior half, of which the cells are large, non-flagellate, and rounded. These two kinds of cell are arranged around a small internal cavity which is largely filled up with amoebocytes. The flagellated cells are invaginated into the dome of rounded cells during metamorphosis, in fact, become the choanocytes or gastral cells; the rounded cells, on the other hand, become the dermal cells—an astonishing fact to any one acquainted only with Metazoan larvae.

A typical parenchymula is that of _Clathrina blanca_ (Fig. 112). When hatched it consists of a wall surrounding a large central cavity and built up of flagellated cells interrupted at the hinder pole by two cells (_p.g.c_)—the mother-cells of archaeocytes. Before the metamorphosis, certain of the flagellated cells leave the wall and sink into the central cavity, and undergoing certain changes establish an inner mass of future dermal cells. By subsequent metamorphosis the remaining flagellated cells become internal, not this time by invagination, but by the included dermal cells breaking through the wall of the larva, and forming themselves into a layer at the outside.

In the larva of _C. blanca_, after a period of free-swimming existence, the same three elements are thus recognisable as in that of _Sycon_ at the time of hatching; in the newly hatched larva of _C. blanca_, however, one set of elements, the dermal cells, are not distinguishable. The difference, then, between the two newly hatched larvae is due to the earlier cell differentiation of the _Sycon_ larva.[254]

Now consider the larva of _Leucosolenia_. It is hatched as a {228}completely flagellated larva; its archaeocytes are internal (as in _Sycon_); future dermal cells, recognisable as such, are absent. They arise, as in _C. blanca_, by transformation of flagellated cells; but (1) this process is confined to the posterior pole, and (2) the internal cavity is small and filled up with archaeocytes. Consequently the cells which have lost their flagella and become converted into dermal cells cannot sink in as in _C. blanca_: they accumulate at the hinder pole, and thus arises a larva half flagellated, half not; in fact, an amphiblastula. Or, briefly, in _Leucosolenia_ the larva at hatching is a parenchymula, and when ready to fix is an amphiblastula; and, again, the difference between the newly hatched larva and that of _Sycon_ is due to the earlier occurrence of cell differentiation in the latter. What completer transitional series could be desired?

Turning to the Micromastictora, the developmental history already sketched is fairly typical (p. 172). The differences between Mega- and Micro-mastictoran larvae are referable mainly to the fact that the dermal cells in the latter become at once differentiated among themselves to form the main types of dermal cell of the adult.[255] The metamorphosis is comparable to that of _C. blanca_. Among Tetractinellida and Hexactinellida sexually produced larvae have not been certainly identified.

Asexual reproduction takes place according to one of three types, which may be alluded to as (1) "budding," (2) "gemmulation," (3) formation of "asexual larvae."

By BUDDING (Fig. 113) is meant the formation of reproductive bodies, each of which contains differentiated elements of the various classes found in the parent. A simple example of this is described by Miklucho Maclay in Ascons, where the bud is merely the end of one of the Ascon tubes which becomes pinched off and so set free.

In _Leucosolenia botryoides_[256] Vasseur describes a similar process; in this, however, a strikingly distinctive feature is present (Fig. 114), namely, the buds have an inverse orientation with respect to that of the parent, so that the budding sponge presents a contrast to a sponge in which multiplication of oscula has occurred. In fact, the free distal end of the bud becomes the base of the young sponge, and the osculum is formed at the opposite extremity, where the bud is constricted from the parent.

{229}

Such a reversal of the position of the bud is noteworthy in view of its rarity, and the case is worth reinvestigating, for in other animal groups a bud or a regenerated part retains so constantly the same orientation as the parent that Loeb,[257] after experimenting on the {230}regeneration of Coelenterata and other forms, concluded that a kind of "polarity" existed in the tissues of certain animals.

In _Oscarella lobularis_[258] the buds are transparent floating bladders, derived from little prominences on the surface of the sponge. Scattered in the walls of the bladders are flagellated chambers, which open into the central cavity. The vesicular nature of the buds is doubtless an adaptation, lessening their specific gravity and so enabling them to float to a distance from the parent.

GEMMULATION.—_Spongilla_ has already afforded us a typical example of this process. Gemmules very similar to those of _Spongilla_ are known in a few marine sponges, especially in _Suberites_ and in _Ficulina_. They form a layer attached to the surface of support of the sponge—a layer which may be single or double, or even three or four tiers deep. A micropyle is sometimes present in the spongin coat, sometimes absent; possibly its absence may be correlated with the piling of one layer of gemmules on another, as this, by covering up the micropyle, would of course render it useless. Presumably when a micropyle is present the living contents escape through it and leave the sponge by way of the canal system (Fig. 115).

The only case besides _Spongilla_ in which the details of development from gemmules have been traced is that of _Tethya_.[259] Mere microscopic examination of a _Tethya_ in active reproduction would suggest that the process was simple budding, but Maas has shown that the offspring arise from groups of archaeocytes in the cortex, that is to say, they are typical gemmules. As they develop they migrate outwards along the radial spicule-bundles {231}and are finally freed, like the buds of the Hexactinellid _Lophocalyx_ (Fig. 113).

The comparison of the process of development on the one hand by gemmules, and on the other by larval development, is of some interest.[260] In both cases two cell layers—a dermal and a gastral—are established before the young sponge has reached a functional state. Differences of detail in the formation of the chambers occur in the gemmule; these find parallels in the differences in the same process exhibited by the larvae of various groups of sponges. On the other hand, the order of tissue differentiation is not the same in the gemmule as in the larva.

Of the reproduction of Tetractinellida extremely little is known. Spermatozoa occur in the tissues in profusion and are doubtless functional, but larvae have been seldom observed.

In Hexactinellida the place of sexually produced larvae is taken by bodies of similar origin to gemmules but with the appearance of parenchymulae. Ijima has indeed seen a few egg-cells in Hexactinellids.[261] He finds, however, that archaeocyte congeries occur in abundance, and there is good reason to believe with him that these are responsible for the numerous parenchymula-like ASEXUALLY PRODUCED LARVAE he has observed. The discovery of "asexual larvae" was first made by Wilson in the Monaxonid _Esperella_; in this case the asexual larva is, as far as can be detected, identical with that developed from the fertilised egg. A similar phenomenon, the production {232}of apparently identical larvae by both sexual and asexual methods, has been observed in the Coelenterate _Gonionema murbachii_.[262]

Artificially, sponges may be reproduced with great advantage to commerce by means of cuttings. Cuttings of the bath sponge are fit to gather after a seven years' growth.

The development of the various forms of SPICULES is a subject about which little is yet known. Most spicules of which the development has been traced originate in a single dermal cell. The triradiate and quadriradiate spicules of Homocoela (Clathrinidae), as Minchin[263] has most beautifully shown, form an exception. Three cells co-operate to form the triradiate; these three divide to give six before the growth of the spicule is complete. A quadriradiate is formed from a triradiate spicule by addition of the fourth ray, which, again, has a separate origin in an independent cell, in fact a porocyte. The triradiate spicules of the Sycettidae, on the other hand, originate in a single cell,[264] but the quadriradiate spicules are formed from these by the addition of a fourth ray in a manner similar to that which has just been described for Clathrinidae.

Monaxon spicules if not of large size undergo their entire development within a single scleroblast (Fig. 118, A). In some cases if their dimensions exceed certain limits, several cells take part in their completion; some of these are derived from the {233}division of the original scleroblast, others are drawn from the surrounding tissue. In _Tethya_, for example, and in _Leucosolenia_[265] the scleroblasts round the large monaxon spicules are so numerous as to have an almost epithelioid arrangement.

The large oxeas of _Tetilla_, _Stelletta_, and _Geodia_, however, are formed each within a single scleroblast.[266]

Triaenes have been shown[267] to originate as monaxons with one swollen termination, from which later the cladi grow out. Information as to the scleroblasts in this case is needed.

The value of a knowledge of the ontogeny of microscleres might be great. Maas believes that he has shown that the spherasters of _Tethya_ are formed by the union of minute tetractine calthrops (Fig. 119, A). If this view should be confirmed, it would afford a very strong argument for the Tetractinellid affinities of _Tethya_.

Keller,[268] on the other hand, finds that the spherasters of the Tetractinellid _Chondrilla_ {234}originate as spheres (Fig. 119, B); and spheres have been observed in the gemmule of a _Tethya_; no spherasters were as yet present in the gemmule, and spheres were absent in the adult.[269]

In the genus _Placospongia_ certain spicules are present which outwardly closely resemble the sterrasters so characteristic of certain Tetractinellidae. Their development, however, as will be seen from Fig. 120, shows that they are not polyaxon but spiny monaxon spicules. _Placospongia_ is consequently transferred to the Monaxonida Spintharaphora.

Sterrasters originate within an oval cell as a number of hairlike fibres[270] (trichites), which are united at their inner ends. The outer ends become thickened and further modified. The position occupied by the nucleus of the scleroblast is marked in the adult spicule by a hilum.

The anisochela has been shown repeatedly to originate from a C-shaped spicule.[271]

What little is known of the development of Hexactinellid spicules we owe to Ijima.[272] Numerous cells are concerned in certain later developmental stages of the hexaster; a hexaster passes through a hexactin stage, and—a fact "possibly of importance for the phylogeny of spicules in Hexactinellida"—in two species the first formed spicules are a kind of hexactin, known as a "stauractin," and possessing only four rays all in one plane (cf. _Protospongia_, p. 207).

PHYSIOLOGY

PRODUCTION OF THE CURRENT.—It is not at first sight obvious that the lashing of flagella in chambers arranged as above {235}described, between an inhalant and an exhalant system of canals, will necessarily produce a current passing inwards at the ostia and outwards at the osculum. And the difficulty seems to be increased when it is found[273] that the flagella in any one chamber do not vibrate in concert, but that each keeps its own time. This, however, is of less consequence than might seem to be the case. Two conditions are essential to produce the observed results: (1) in order that the water should escape at the mouth of the chamber there must be a pressure within the chamber higher than that in the exhalant passages; (2) in order that water may enter the chamber there must be within it a pressure less than that in the inhalant passages. But the pressure in the inhalant and exhalant passages is presumably the same, at any rate before the current is started, therefore there must be a difference of pressure within the chamber itself, and the less pressure must be round the periphery. Such a distribution of pressures would be set up if each flagellum caused a flow of water directed away from its own cell and towards the centre of the chamber; and this would be true whether the flagellum beats synchronously with its fellows or not.

The comparative study of the canal systems of sponges[274] acquires a greater interest in proportion as the hope of correlating modifications with increase of efficiency seems to be realised. In a few main issues this hope may be said to have been realised. The points, so to speak, of a good canal system are (1) high oscular velocity, which ensures rapid removal of waste products to a wholesome distance; (2) a slow current without eddies in the flagellated chambers, to allow of the choanocytes picking up food particles (see below), and moreover to prevent injury to the delicate collars of those cells; (3) a small area of choanocytes, and consequent small expenditure of energy in current production.

It is then at once clear at what a disadvantage the Ascons are placed as compared with other sponges having canal systems of the second or third types. Their chamber and oscular currents can differ but slightly, the difference being obtained merely by narrowing the lumen of the distal extremity of the body to form the oscular rim. Further, the choanocytes are {236}acting on a volume of water which they can only imperfectly control, and it is no doubt due to the necessity of limiting the volume of water which the choanocytes have to set in motion that the members of the Ascon family are so restricted in size. The oscular rim is only a special case of a device adopted by sponges at the very outset of their career, and retained and perfected when they have reached their greatest heights; the volume of water passing per second over every cross-section of the path of the current is of course the same, therefore by narrowing the cross-sectional area of the path at any point, the velocity of the current is proportionally increased at that point. The lining of the oscular rim is of pinacocytes; they determine a smooth surface, offering little frictional resistance to the current, while choanocytes in the same position would have been a hindrance, not only by setting up friction, but by causing irregularities in the motion.

Canal systems of the second type show a double advance upon that of the Ascons, namely, subdivision of the gastral cavity and much greater length of the smooth walled exhalant passage. The choanocytes have now a task more equal to their strength, and, further, there is now a very great inequality between the total sectional areas of the flagellated chambers and that of the oscular tube.

Canal systems of the third type with tubular chambers are an improvement on those of the second, in that the area of choanocytes is increased by the pouching of the chamber-layer without corresponding increase in the size of the sponge. However, the area of choanocytes represents expenditure of energy, and the next problem to be solved is how to retain the improved current and at the same time to cut down expense. The first step is to change the form of the chamber from tubular to spherical. Now the energy of all the choanocytes is concentrated on the same small volume of water. The area of choanocytes is less, but the end result is as good as before. At the wide mouth of the spherical chamber there is nevertheless still a cause of loss of energy in the form of eddies, and it is as an obviation of these that one must regard the aphodi and prosodi with which higher members of the Demospongiae are provided. The correctness of this view receives support, apart from mechanical principles, from the fact that the mass of the body of any one of these sponges is greater relatively to the total flagellated area than in those sponges with eurypylous chambers; that is to say, a few {237}aphodal and diplodal chambers are as efficient as many of the eurypylous type.

It is manifest that the current is the bearer of the supply of FOOD; but it requires more care to discover (1) what is the nature of the food; (2) by which of the cells bathed by the current the food is captured and by which digested. The answer to the latter question has long been sought by experimenters,[275] who supplied the living sponge with finely powdered coloured matters, such as carmine, indigo, charcoal, suspended in water. The results received conflicting interpretations until it became recognised that it was essential to take into account the length of time during which the sponge had been fed before its tissues were subjected to microscopic examination. Vosmaer and Pekelharing obtained the following facts: _Spongilla lacustris_ and _Sycon ciliatum_, when killed after feeding for from half an hour to two hours with milk or carmine, contain these substances in abundance in the bodies of the choanocytes and to a slight degree in the deeper cells of the dermal tissue; after feeding for twenty-four hours the proportions are reversed, and if a period of existence in water uncharged with carmine intervenes between the long feed and death then the chambers are completely free from carmine. These are perhaps the most conclusive experiments yet described, and they show that the choanocytes ingest solid particles and that the amoeboid cells of the dermal layer receive the ingested matter from them. In all probability it is fair to argue from these facts that solid particles of matter suitable to form food for the sponge are similarly dealt with by it and undergo digestion in the dermal cells.

Choanocytes are the feeding organs _par excellence_; but the pinacocytes perform a small share of the function of ingestion, and in the higher sponges where the dermal tissue has acquired a great bulk the share is perhaps increased.

In the above experiments is implied the tacit assumption that sponges take their food in the form of finely divided solids. Haeckel[276] states his opinion that they feed on solid particles derived from decaying organisms, but that possibly decaying substances in solution may eke out their diet. Loisel, in 1898,[277] {238}made a new departure in the field of experiment by feeding sponges with coloured solutions, and obtained valuable results. Thus solutions, if presented to the sponge in a state of extreme dilution, are subjected to choice, some being absorbed, some rejected. When absorbed they are accumulated in vacuoles within both dermal and gastral cells, mixed solutions are separated into their constituents and collected into separate vacuoles. In the vacuoles the solutions may undergo change; Congo red becomes violet, the colour which it assumes when treated with acid, and similarly blue litmus turns red. The contents of the vacuoles, sometimes modified, sometimes not, are poured out into the intercellular gelatinous matrix of the dermal layer, whence they are removed partly by amoeboid cells, partly, so Loisel thinks, by the action of the matrix itself. It adds to the value of these observations to learn that Loisel kept a _Spongilla_ supplied with filtered spring-water, to which was added the filtered juice obtained from another crushed sponge. This _Spongilla_ lived and budded, and was in good health at the end of ten days.

MOVEMENT.—Sponges are capable of locomotion only in the young stage; in the adult the only signs of movement are the exhalant current, and in some cases movements of contraction sufficiently marked to be visible to the naked eye. Meresjkowsky was one of the early observers of these movements. He mentions that he stimulated a certain corticate Monaxonid sponge by means of a needle point: a definite response to each prick inside the oscular rim was given by the speedy contraction of the osculum.[278]

PIGMENTS AND SPICULES.—Various reasons lead one to conclude that the spicules have some function other than that of support and defence, probably connected with metabolism. For the spicules are cast off, sometimes in large numbers, to be replaced rapidly by new ones, a process for which it is difficult to find an adequate explanation if the spicules are regarded as merely skeletal and defensive.[279] Potts remarks upon the striking profusion with which spicules are secreted by developing Spongillids from water in which the percentage of silica present must have been exceedingly small. The young sponges climbed {239}up the strands of spicules as they formed them, leaving the lower parts behind and adding to the upper ends.

Of the physiology of the pigments of sponges not much is yet known: a useful summary of facts will be found in Von Fürth's text-book.[280]

SPONGIN.—Von Fürth[280] points out that this term is really a collective one, seeing that the identity of the organic skeletal substance of all sponge species is hardly to be assumed. Spongin is remarkable for containing iodine. The amount of iodine present in different sponges varies widely, reaching in certain tropical species of the Aplysinidae and Spongidae the high figure 8 to 14 per cent. Seaweeds which are specially rich in iodine contain only 1.5 to 1.6 per cent.

In view of the fact that iodine is a specific for croup, it is of interest to observe that the old herb doctors for many centuries recognised the bath sponge as a cure for that disease.

DISTRIBUTION IN SPACE.—All the larger groups of Sponges are cosmopolitan. Each group has, however, its characteristic bathymetrical range: the facts are best displayed by means of curves, as in Fig. 122, which is based wholly on the results obtained by the "Challenger" Expedition. The information as to littoral species is consequently inadequate, and we have not the data requisite for their discussion.

Sponges generally (_a_) and Monaxonida in particular (_b_) are more generally distributed in water of depths of 51 to 200 fathoms than in depths of less than 50 fathoms; but localities in shallow water are {240}richer, for the station curve (_a'_) rises abruptly from I. to II., while the species curve (_a_) in the same region is almost horizontal.

The Hexactinellid curve (_c_) culminates on III., showing that the group is characteristically deep water. That for Tetractinellida (_d_) reaches its greatest height on II., _i.e._ between 51 and 200 fathoms. Even here, in their characteristic depths, the Tetractinellida fall below the Hexactinellida, and far below the Monaxonida in numbers. Again, the Monaxonida are commoner than Hexactinellida in deep water of 201 to 1000 fathoms, and it is not till depths of 1000 fathoms are passed that Hexactinellida prevail, finally preponderating over the Monaxonida in the ratio of 2:1.

The Calcarea and Ceratosa are strictly shallow-water forms. It is a fact well worth consideration that the stations at which sponges have been found are situated, quite irrespective of depth, more or less in the neighbourhood of land. In the case of Calcarea and Ceratosa this is to be expected, seeing that shallow water is commonest near land, but it is surprising that it should be true also of the Hexactinellida and of the deep-water species of Tetractinellida and of Monaxonida.

While the family groups are cosmopolitan, this is not true of genera and species. The distribution of genera and species makes it possible to define certain geographical provinces for sponges as for other animals. That this is so, is due to the existence of ocean tracts bare of islands; for ocean currents, can act as distributing agents with success only if they flow along a coast or across an ocean studded with islands. It is, of course, the larval forms which will be transported; whether they will ever develop to the adult condition depends on whether the current carrying them passes over a bottom suitable to their species before metamorphosis occurs and the young sponge sinks. If such a bottom is passed over, and if the depth is one which can be supported by the particular species in question, then a new station may thus be established for that species.

The distance over which a larva may be carried depends on the speed of the current by which it is borne, and on the length of time occupied by its metamorphosis. Certain of the ocean currents accomplish 500 miles in six days; this gives some idea of the distance which may intervene between the birthplace and {241}the final station of a sponge; for six days is not an excessive interval to allow for the larval period of at any rate some species.

DISTRIBUTION IN TIME.—All that space permits us to say on the palaeontology of sponges has been said under the headings of the respective classes. We can here merely refer to the chronological table shown in Fig. 123:[281]—

FLINTS.—The ultimate source of all the silica in the sea and fresh-water areas is to be found in the decomposition of igneous rocks such as granite. The quantity of silica present in solution in sea water is exceedingly small, amounting to about one-and-a-half parts in 100,000; it certainly is not much more in average fresh water. This is no doubt due to its extraction by diatoms, which begin to extract it almost as soon as it is set free from the parent rock. It is from this small quantity that the siliceous sponges derive the supply from which they form their spicules. Hence it would appear that for the formation of one {242}ounce of spicules at least one ton of sea water must pass through the body of the sponge. Obviously from such a weak solution the deposition of silica will not occur by ordinary physical agencies; it requires the unexplained action of living organisms. This may account for the fact that deposits of flint and chert are always associated with organic remains, such as Sponges and Radiolaria. By some process, the details of which are not yet understood, the silica of the skeleton passes into solution. In Calcareous deposits, a replacement of the carbonate of lime by the silica takes place, so that in the case of chalk the shells of Foraminifera, such as _Globigerina_ and _Textularia_ and those of Coccoliths, are converted into a siliceous chalk. Thus a siliceous chalk is the first stage in the formation of a flint.

A further deposition of silica then follows, cementing this pulverulent material into a hard white porous flint. It is white for the same reason that snow is white. The deposition of silica continues, and the flint becomes at first grey and at last apparently black (black as ice is black on a pond). Frequently flints are found in all stages of formation: siliceous chalk with the corroded remains of sponge spicules may be found in the interior, black flint blotched with grey forming the mass of the nodule, while the exterior is completed by a thin layer of white porous flint. This layer must not be confused with the white layer which is frequently met with on the surface of weathered flints, which is due to a subsequent solution of some of the silica, so that by a process of unbuilding, the flint is brought back to the incompleted flint in its second stage. In the chalk adjacent to the flints, hollow casts of large sponge spicules may sometimes be observed, proving the fact, which is however unexplained, of the solution of the spicular silica. The formation of the flints appears to have taken place, to some extent at least, long after the death of the sponge, and even subsequent to the elevation of the chalk far above the sea-level, as is shown by the occurrence of layers of flints in the joints of the solid chalk.[282]

COELENTERATA AND CTENOPHORA

BY

S. J. HICKSON, M.A., F.R.S.

Formerly Fellow and now Honorary Fellow of Downing College, Beyer Professor of Zoology in the Victoria University of Manchester.

{245}CHAPTER X

COELENTERATA