BY J. WALTER FEWKES.

(Continued from October number, 1881.)

IV. ANATOMY AND DEVELOPMENT OF DIPHYES.

THE Siphonophores which we have thus far considered all agree in this particular, that they have a float attached at one end of the stem to buoy it up in the water. It may, in some genera, be doubtful how far this structure is necessary, or to what extent it is functional, but it is never without representation in any of the Physophorida. We come now to consider tubular jelly-fishes, which may be looked upon as in many respects the highest of the Siphonophores. In no member of the group is there a float such as is to be found in Agalma and its allies, while in details of structure their organization is very characteristic, and different from the tubular Medusæ already studied. A good representative of these Medusa, whose several genera make up the Diphyidæ, is the beautiful genus Diphyes, represented in our waters, as far as explored, by a single species. An account of the anatomy and development of this genus seems a fitting introduction to a more extended acquaintance with the remaining Siphonophores, which embrace some of the most beautiful animals with which the student of marine life is familiar.

The differences between Diphyes and Agalma seem so great

1 If we consider, however, their anatomy, and the likeness of some of the Diphyida to the primitive medusa of Agalma, we may place them, as a whole, below the Physophoridae. My reasons for placing them higher will be given later in this

paper.

2 The designation Diphyidæ seems to me preferable to Leuckart's term, Calycophoride. The very aberrant genus Hippopodius is the type of a family between the Physophorida and Diphyidæ.

that, at first sight, it is almost impossible to recognize anything in common between them both. A more intimate study, however,

FIG. 1.-Diphyes formosa, sp. nov. a, covering scale; ab, anterior bell; bc, bell cavity; pb, posterior bell; ps, pigment spot (ocellus ?); c, long tube of anterior bell; 1, somatocyst; n, ridges on lower side of posterior bell; s, stem; a covering-scale; P, polypite; i, tentacle.

brings out very many resemblances which a casual observation had overlooked.

Prominent among all the structures which characterize the Siphonophores, is the axis or stem from which the group is named. In Diphyes this part (s) is very well developed, and in live specimens may be seen trailing behind to a great distance in the water, just as we have seen was the case in the genera of Physophoridae already mentioned. Along its whole extent we find appendages so fastened that they do not incommode in the least possible manner the direct motion of the animal through the water. In the genus Diphyes it will be noticed that all the organs are especially adapted for rapid motion, and as one watches these graceful tubes, with their appendages, shooting through the sea, the adaptation for this mode of life seems complete. With this thought in mind, one can almost predict the organs of the Physophores which should be missed in Diphyes, and the modifications of their form which would be expected.

A float would, if of any size, be a great impediment to the free motion of the jelly-fish. In Diphyes, consequently, it is altogether wanting, and other methods are resorted to in order to diminish the specific gravity of the colony.

No organ of Diphyes better illustrates the modification and adaptation which has taken place to bring about rapid motion, than those which move the colony, which are here, as in all Siphonophores, the nectocalyces. There are only two of these swimming-bells, as they are called, and they are very different in outline and general appearance from the swimming-bells of any of the animals which we have yet considered. These bells differ also one from the other, in size, shape and anatomy.

At one end of the axis of Diphyes, as it floats gracefully extended in the water, there are two gelatinous, transparent bodies of somewhat conical shape (ab, pb); these are the two nectocalyces which, with the exception of one genus, Hippopodius, are double throughout all the members of the Diphyidæ.1 The connection between the two bells at the extremity of the axis is so strong, that when they are raised from the water they are not broken apart, but the axis, by contraction, is simply drawn up into a deep groove in the under side of the bell, while the appendages, even when the colony is lifted out of water, remain attached

In the genus Monophyes there is but a single nectocalyx. This genus is, in this respect as well as in other details of structure, very peculiar. I shall speak of it more at length in considering the different genera of Diphyide. Hippopodius has many nectocalyces.

much more strongly than corresponding structures of Agalma and kindred forms. In this retracted position they are often carried, as the animal darts forward in its course through the water. To facilitate that motion by diminishing the resistance of the surrounding medium, the method of attachment no less than the form of the bells, contributes.

In Agalma the nectocalyces, as we have seen, seem to arise in two rows, with bell openings looking in opposite directions. They are capable of a very limited change of position, and most of the variety of motion which the colony has, is brought about by combination in the action of nectocalyces situated in different regions of the stem, or in a muscular twisting of the axis upon which they are fastened, by which their openings are made to face in different directions. The method is too simple if rapid motion be desired, and ill adapted to that purpose in Diphyes. In Stephanomia variety and rapidity of movement are brought about by multiplication of nectocalyces. Even in this genus the means are inferior to those which we find in Diphyes.1

The swimming-bells of Diphyes are placed one behind the other, so that their longer axes lie in a straight line which falls in the direction of motion. Both bell cavities open in the same way, facing backward as they float in the water. When they act simultaneously the fluid ejected from their cavities by the contraction of the bell walls, presses together on the surrounding medium and reinforces each other. There is no action of one bell in opposition to another, as may happen in Agalma. The volume of ejected water is comparatively much larger than in any of the Physophorida.

The anterior bell (ab) of the two nectocalyces has a pyramidal shape, and is pointed at the apex opposite the bell opening. If this bell were attached by the same region as the nectocalyx of Agalma, it would seem as if this apical prolongation should also indicate the place of attachment of the stem. In Diphyes, however, this is not the case. The apex of the first bell is not homologous to the apex of the bell of other Medusa, nor does it correspond to the point of attachment of the nectocalyx to the stem

1 The motion of the Diphyes is sometimes so rapid that the eye cannot follow the animal. The water is driven out of the bell cavity by a single muscular contraction of the bell walls and when the impetus is lost a new contraction follows. movement of the two nectocalyces is simultaneous.

The

of Agalma. The apex of the anterior bell is in reality the prolongation of the side of the bell, while the true apex has been abnormally twisted out of position, and is found just above the bell opening, near the origin of the stem which seems to hang down between the two nectocalyces.

Nowhere in its structure is the modification, which takes place in the organs of the bell as a result of this abnormal twisting, better shown than in the course of the chymiferous tubes upon the inner walls of the bell cavity. The radial tubes are especially modified in their course by the change in external form which the bell has undergone.

The chymiferous tubes of the anterior nectocalyx in Diphyes, consist of a system of four radial vessels placed upon the inner walls of the bell cavity, and a single large tube or cavity extending into one side of the bell walls parallel to the outer surface. The former tubes start from a common junction, and pass radially to the bell margin, while the latter ends blindly about two-thirds the distance between the bell rim and the pointed extremity of the nectocalyx. Both open into the cavity of the stem; the former by a vessel passing from their junction to the stem; the latter more directly through the same tube.

The length of the four radial tubes is very unequal, as would naturally be expected if the distortion which we have suggested as having taken place in the anterior bell, has in reality occurred. The two tubes (c) which lie in those parts of the bell which have been enlarged, are therefore naturally much longer than those in the remaining portions of the bell. So small indeed has that side of the nectocalyx which adjoins the posterior bell become, and so enormously has the opposite half been enlarged, that the tubes of one are inconspicuous and with difficulty traced, while those of the other are very prominent on the inner bell walls. At first sight then, we might suppose that there were but two radial vessels, while a closer study shows that there are four such tubes as we have seen exists in the nectocalyces of all Siphonophores. At the common junction of these tubes, we must look for the apex of the bell cavity. At that point, about midway in the length of the two bells, the vessels communicate with the stem cavity by means of a short tube, similarly placed to a like vessel in the nectocalyx of Agalma.

There is, however, in the anterior nectocalyx a tube (1) which