Seaside Studies in Natural History Part 6

This is another Tubularian, covering the surface of rocks in tide-pools, or attaching itself upon sh.e.l.ls inhabited by hermit crabs.

Indeed it was upon these sh.e.l.ls that the Hydractinia was first noticed, and it was long supposed that the wanderings to which the little colony was thus subjected were necessary for its healthy development. But subsequent observations have shown that it attaches itself quite as frequently to the solid rock as to these nomadic sh.e.l.ls. It has a rosy color, and, being very small, it looks, until one examines it closely, more like a thick red carpet of soft moss, than like a colony of animals. These communities are distinct in s.e.x, the fertile individuals in each being either all male or all female.

In Fig. 100 we have a portion of a female colony, representing one fertile head, in which the buds are crowded with Medusae; one sterile head, surrounded by its wreath of tentacles; and still another member of the society whose office is not fully understood, unless it be that of a kind of purveyor, catching food for the rest. Fig. 101 represents the corresponding individuals taken from a male colony. The s.e.x makes little difference in the appearance of the reproductive heads. All the individuals of a Hydractinia colony are connected at the base by a h.o.r.n.y network, rising occasionally into points of a conical or cylindrical shape. This polymorphism among the Tubularians is another evidence of the relation between the Siphonophorae, or floating Hydroids, and the fixed Hydroids.

[Ill.u.s.tration: Fig. 100. Female colony of Hydractinia; _a_ sterile individual, _b_ fertile individual producing female Medusae, _c_ fertile individual with globular tentacles without Medusae, _d_ _e_ _f_ _g_ _h_ _i_ Medusae in different stages of growth, _o_ mouth tentacles. (_Aga.s.siz_.)]

[Ill.u.s.tration: Fig. 101. Male colony; _a_ _a_ sterile individuals, _b_ fertile individuals producing male Medusae, _d_; _o_ globular tentacles, _t_ slender tentacles of sterile individual.

(_Aga.s.siz_.)]

_Hybocodon_. (_Hybocodon prolifer_ AG.)

Among our Medusae derived from a Tubularian stock is the Hybocodon, viz. the hunchbacked Medusa (Fig. 102), a singular little Jelly-fish, odd and unsymmetrical in shape, as its name indicates, and interesting from its relations to one of our floating communities, the Nanomia, presently to be described. Instead of the evenly proportioned bell of the ordinary Medusae, the Hybocodon has a one-sided outline (Fig. 102), one large tentacle only being fully developed, while the others remain always abortive, so that the whole weight of the structure is thrown on one half of the bell. Upon this large tentacle small Jelly-fishes, similar to the original, are produced by budding, this process going on till ten or twelve such Jelly-fishes (Fig. 103) may be seen suspended from the tentacle. Up to this time it has remained connected with the Hydroid from which it arises, a rather large Tubularian, usually growing singly (Fig. 104), and of a deep orange-red in color.

But at this stage of its existence it frees itself, and leads an independent life hereafter, swimming about with a quick, darting motion. In the account of the Nanomia, the h.o.m.ology between its scale, or abortive Medusa, and the Hybocodon, is traced in detail, and I need only allude to it here. Though this Medusa is so peculiar in appearance, the Tubularian from which it is derived is very like the _Tubularia Couthouyi_, already described. This is one of the instances before alluded to, in which closely allied forms give rise to very dissimilar ones, or, as in many cases, the very reverse of this takes place, and closely allied forms arise from very dissimilar ones.

[Ill.u.s.tration: Fig. 102. Unsymmetrical free Medusa of Hybocodon; _r_ _o_ chymiferous tubes, _v_ proboscis, _s_ circular tube, _m_ young Medusae at base of long tentacle _t_. (_Aga.s.siz_.)]

[Ill.u.s.tration: Fig. 103. Medusa bud of Hybocodon; _a_ base of attachment, _o_ proboscis, _c_ circular tube, _d_ young Medusae at base of long tentacle _t_. (_Aga.s.siz_.)]

[Ill.u.s.tration: Fig. 104. Single head of Hybocodon Hydroid; _a_ stem, _d_ Medusae buds, _o_ tentacles round mouth. (_Aga.s.siz_.)]

_Dysmorphosa_. (_Dysmorphosa fulgurans_ A. AG.)

Besides the budding at the base of the tentacle, as in Hybocodon, we find another mode of development among Hydroid Medusae, viz. that of budding from the proboscis. One of our most common little Jelly-fishes, the Dysmorphosa (Fig. 105), to which we owe the occasional blue phosph.o.r.escence of the sea, so brilliant at times, buds in this manner. Fig. 105 represents an adult Dysmorphosa, on the proboscis of which may be seen three small buds in different stages of development. In Fig. 106 the proboscis is more enlarged, showing one of the little Jelly-fishes similar to the parent, just ready to drop off. We need not wonder at the immense number of these animals, with which the sea actually swarms at times, when we know that as fast as they are dropped, and it takes but a few days to complete their development, they each begin the same process; so that in the course of a week or ten days one such Medusa, supposing it to have produced six buds only, will have given rise to forty-two Jelly-fishes, thirty-six of which may be equally prolific in the same short period.

These Medusae budding thus, and swimming about, carrying their young with them, bear such a close resemblance to the floating communities of Hydroids formerly known as Siphonophorae, that did we not know that some of them arise from Tubularians, it would be natural to a.s.sociate them with the Siphonophorae.

[Ill.u.s.tration: Fig. 105. Dysmorphosa seen in profile; magnified.]

[Ill.u.s.tration: Fig. 106. Magnified proboscis of Dysmorphosa with young Medusae budding from it.]

_Nanomia_. (_Nanomia cara_ A. AG.)

The Nanomia (Fig. 115), our free floating Hydroid, consists, when first formed, of a single Hydra containing an oblong oil bubble (Fig.

107). The whole organisation of such a Hydra is limited to a simple digestive cavity; it has, in fact, but one organ, and one function, and consists of an alimentary sac resembling the proboscis of a Medusa (Fig. 107); the oil bubble is separated from it by a transverse part.i.tion, and has no connection with the cavity. Presently, between the oil bubble and the cavity arise a number of buds of various character (Fig. 108), which we will describe one by one, beginning with those nearest the oil bubble, since these upper members of the little swimming community bear a very important part in its history.

The infant community (Fig. 108) pa.s.ses rapidly into the stage represented in Fig. 109, and then through all the stages intermediate between this and the adult, shown in its natural size in Fig. 115. The upper buds enlarge gradually, and soon take upon themselves a perfect Medusa structure (Fig. 110), with the exception of the proboscis, the absence of which is easily understood, when we find that these Medusae, serve the purpose of locomotion only, having no share in the function of feeding the community, so that a digestive apparatus would be quite superfluous for them. In every other respect they are perfect Medusae, attached to the Hydra as the Medusa buds always are when first formed, having the (four) chymiferous tubes, characteristic of all Hydroid Medusae, radiating from the centre to the periphery; two of these tubes are very winding, as may be seen in Fig. 110, while the other pair are straight. The Medusae themselves are heart-shaped in form, depressed at the centre of the upper surface, and bulging on either side into wing-like expansions, where they join the stem. These expansions interlock with one another, crossing nearly at right angles. The Medusae-like buds are the swimming bells; by their contractions, alternately taking in and throwing out the water, they impel the whole community forward, so that it seems rather to move like one animal, than like a combination of individuals.

[Ill.u.s.tration: Fig. 107. Young Nanomia; magnified.]

[Ill.u.s.tration: Fig, 108. Young Nanomia with rudimentary Medusae.]

[Ill.u.s.tration: Fig. 109. Young Nanomia, older than Fig. 108.]

[Ill.u.s.tration: Fig. 110. Heart-shaped swimming bell of Nanomia; magnified.]

Besides these locomotive members, the community contains three kinds of Hydrae arising as buds from the primitive Hydra below the swimming bells, the latter remaining always nearest the oil bubble at the top, while the first Hydra, the founder of the community, in proportion as the new individuals are added, is gradually pushed downward, and remains always at the end of the string, the stem of which is formed by the elongated neck of the primitive Hydra. All the three sets of Hydrae have certain features in common, while they have other distinguis.h.i.+ng characteristics marking them as distinct individuals.

They are all accompanied by triangular s.h.i.+elds (Fig. 111), arising with them at the same point on the parent stem, and all are furnished with tentacles hanging down from the summit of the Hydra at the side opposite the s.h.i.+eld. These facts are important to remember, since we shall presently perceive, upon a.n.a.lyzing their parts, that these Hydrae have a close h.o.m.ology to the Hybocodon. The tentacles differ in structure as well as in number for each kind of Hydra. Having shown in what characters they agree, let us now take each set individually, and see what differences they present.

[Ill.u.s.tration: Fig. 111. Cl.u.s.ter of Medusae with tentacles having pendent k.n.o.bs.]

In the first set which we will examine the Hydra is open-mouthed. Like the original Hydra, it is only a digestive tube, similar in all respects to the proboscis of a Medusa-disk. Its only function is that of feeding, and it shows a laudable fidelity to its calling, being very constantly and earnestly engaged in the work. Let us add, however, that in this instance the occupation is not a wholly selfish one, since the cavity of every Hydra communicates with that of the stem, and the food taken in at these over-gaping mouths, is at once circulated through all parts of the community, with the exception of the oil bubble, from which it is excluded by the transverse part.i.tion dividing it from all the lower members of the stock. The s.h.i.+elds share in this general nourishment of the compound body by means of chymiferous tubes extending toward the outer surface, and opening into the cavity of the stem. The mouth of this Hydra is very flexible (Fig.

111), expanding and contracting at the will of the animal, and sometimes acting as a sucker, fastening itself, leech-like, on the object from which it seeks to draw its sustenance. (See Fig. 111.) The tentacles attached to this set of Hydrae are exceedingly long and delicate. They arise in a cl.u.s.ter at the upper and inner edge of the Hydra, just at its point of juncture with the stem, and being extremely flexible and contractile, their long tendril-like sprays are thrown out in an endless variety of att.i.tudes. (See Fig. 115.) Along the whole length of this kind of tentacle are attached little pendent k.n.o.bs at even distances; Fig. 112 represents such a k.n.o.b greatly magnified, and absolutely paved with la.s.so-cells, the inner and smaller ones being surrounded by a row of larger ones.

[Ill.u.s.tration: Fig. 112. Magnified pendent k.n.o.b.]

[Ill.u.s.tration: Fig. 113. Medusa with corkscrew shaped tentacles.]

The second set of Hydrae (Fig. 113), are also open-mouthed, corresponding with those described above, in everything except the tentacles, which are both shorter and thicker, and are coiled in a corkscrew-like spiral. These are thickly studded for their whole length with la.s.so-cells. (See Fig. 113.)

In the third and last set of Hydrae (Fig. 114), the mouth is closed; they have, therefore, no share in feeding the community, but receive their nourishment from the cavity of the stem into which they open.

They differ also from the others in having a single tentacle instead of a cl.u.s.ter, and on this tentacle the la.s.so-cells are scattered at uneven distances (Fig. 114). The special function of these closed Hydrae is yet to be explained; they have oil bubbles at their upper end (see Fig. 111, the top Hydra), and though we have never seen them drop off, it seems natural to suppose that they do separate from the parent stock, and found new communities similar to those from which they arise.

[Ill.u.s.tration: Fig. 114. Medusa with a simple thread-like tentacle.]

The intricate story of this singular compound existence does not end here. There is still another set of individuals whose share in maintaining the life of the community is by no means the least important. Little bunches of buds, of a different character from any described above, may be seen at certain distances along the lower part of the stem. These are the reproductive individuals. They are cl.u.s.ters of imperfect s.e.xual Medusae, resembling the rudimentary Medusae of Tubularia (Fig. 99), which are never freed from the parent stem, but discharge their contents at the breeding season. Like many other compound Hydroids, the s.e.xes are never combined, in one of these communities; they are always either male or female, and as those with female buds have not yet been observed, we can only judge by inference of their probable character. Front what is already known, however, of Hydroid communities of a like description, we suppose that the process of reproduction must be the same in these, and that the female stocks of Nanomia give birth to small Jelly-fishes, the eggs of which become oil bubbles, similar to that with which our little community began.

(Fig. 108.)

[Ill.u.s.tration: Fig. 115. Adult Nanomia, natural size, at rest.]

By the time all these individuals have been added along the length of the stem, the stem itself has grown to be about three inches long (Fig. 115), though the tentacles hanging from the various members of the community give to the whole an appearance of much greater length.

The motion of this little string of living beings is most graceful.

The oil bubble (Fig. 116) at the upper end is their float; the swimming bells immediately below it (Fig. 110), by the convulsive contractions of which they move along, are their oars. The water is not taken in and expelled again by all the bells at once, but first from all the bells on one side, beginning at the lower one, and then from all those on the opposite side, beginning also at the lower one; this alternate action gives to their movements a swinging, swaying character, expressive of the utmost freedom and grace. Whether such a little community darts with a lightning-like speed through the water, or floats quietly up and down, for its movements are both rapid and gentle, it always sways in this way from side to side. Its beauty is increased by the spots of bright red scattered along the length of the stock at the base and tips of the Hydrae, as well as upon the tentacles. The movements and att.i.tudes of the tentacles are most various. Sometimes they shoot them out in straight lines on either side, and then the aspect of the whole thing reminds one of a tiny chandelier in which the coral drops make the pendants, or they may be caught up in a succession of loops or floating in long streamers; indeed, there is no end to the fantastic forms they a.s.sume, ever astonis.h.i.+ng you by some new combination of curves. The prevailing hue of the whole community is rosy, with the exception of the oil bubble or float, which looks a bright garnet color when seen in certain lights.

[Ill.u.s.tration: Fig. 116. Oil float of Nanomia; greatly magnified.]

Let us now compare one of the Hydrae hanging from the stem (Fig. 113) with the Hybocodon (Fig. 102). The reader will remember the unsymmetrical bell of this singular Medusa, one half of its disk more largely developed than the other, with the proboscis hanging from the centre, and the cl.u.s.ter of tentacles from one side. Let us now split the bell so as to divide it in two halves with the proboscis hanging between them; next enlarge the side where there are no tentacles, and give it a triangular outline; then contract the opposite side so as to draw up the cl.u.s.ter of tentacles to meet the base of the proboscis, and what have we? The proboscis now corresponds to the Hydra of our Nanomia, with the cl.u.s.ter of tentacles attached to its upper edge (Fig. 113), while the enlarged half of the bell represents the s.h.i.+eld.

If this h.o.m.ology be correct it shows that the Nanomia is not, as some naturalists have supposed all the Siphonoph.o.r.es to be, a single animal, its different parts being a mere collection of organs endowed with special functions, as feeding, locomotion, reproduction, &c., but that it is indeed a community of distinct individuals corresponding exactly to the polymorphous Hydroids, whose stocks are attached, such as Hydractinia, and differing from them only in being free and floating.

The h.o.m.ologies of the Siphonophorae or floating Hydroids, with many of the fixed Hydroids, is perhaps more striking when we compare the earlier stages of their growth. Suppose, for instance, that the planula of our Melicertum (See Fig. 81) should undergo its development without becoming attached to the ground,--what should we then have? A floating community (Fig. 83), including on the same stock like the Nanomia, both sterile and fertile Hydrae, from the latter of which Medusae bells are developed. The little Hydractinia community (Fig.

100), in which we have no less than four distinct kinds of individuals, each performing a definite distinct function, affords a still better comparison.

_Physalia_. (_Physalia Arethusa_ TIL.)

[Ill.u.s.tration: Fig. 117. Physalia; _a_ _b_ air sac with crest _c_, _m_ bunches of individuals, _n_ central tentacles, _t_ _t_ expanded tentacles. (_Aga.s.siz_.)]

Among the most beautiful of the Siphonoph.o.r.es, is the well-known Physalia or Portuguese man-of-war, represented in Fig. 117. The float above is a sort of crested sac or bladder, while the long streamers below consist of a number of individuals corresponding in their nature and functions to those composing a Hydroid community. Among them are the fertile and sterile Hydrae (Fig. 118), the feeders and Medusae bells (Fig. 119). The Physalia properly belongs to tropical waters, but sometimes floats northward, in the warm current of the Gulf Stream, and is stranded on Cape Cod. When found so far from their home, however, they have usually lost much of their vividness of color; to judge of their beauty one should see them in the Gulf of Mexico, sailing along with their brilliant float fully expanded, their crest raised, and their long tentacles trailing after them.

[Ill.u.s.tration: Fig. 118. Bunch of Hydrae; _a_ base of attachment, _b_ _b_ _b_ single Hydrae, _c_ _c_ tentacles. (_Aga.s.siz_.)]

[Ill.u.s.tration: Fig. 119. Bunch of Hydrae; cl.u.s.ter of Medusae; _b_ _b_ Hydrae with tentacles, _c_ _d_ bunches of Medusae (_Aga.s.siz_.)]

_Velella_. (_Velella mutica_ BOSC.)

Another very beautiful floating Hydroid, occasionally caught in our waters, though its home is also far to the south, is the Velella (Fig.

120). It is bright blue in color, and in form not unlike a little flat boat with an upright sail. Its Medusa (Fig. 121) resembles so much that of some of our Tubularians, that it has actually been removed on this account from the old group of Siphonophorae, and placed next the Tubularians; another evidence of the close affinity between the former and the Hydroids.