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remote period as the commencement of the eleventh century : that we cannot bring it down to the middle of the seventeenth century (the only possible subsequent date on the above supposition) must be admitted.

That the peculiar period embraced in plates XX-XXIII may be located where any two cycles meet is certainly true, so far as the years are concerned, but judging by the symbols and extent of the period, certain signs which seem to indicate the 3d and 1st Ahau, and from the fact that the commencement of no other cycle, except that with which the grand cycle begins, coincides with the commencement of an Ahau, I am satisfied it marks the union of two of the greatest Maya periods.1

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THE SIPHONOPHORES.

III.—PHYSOPHORIDE (ANIMALS CLOSELY RELATED TO AGALMA.

IN

BY J. WALTER FEWKES.

the two previous articles in the NATURALIST, we have sketched in outline the anatomy and development of Agalma, which is regarded as the typical genus of a family of tubular jelly-fishes to which is given the name of Physophoridæ, in distinction from others yet to be mentioned, which are but distantly related to the type chosen. Before we go further a consideration of the different genera found in this family may be of interest to our readers.

All the genera now to be described agree in this particular, that they have a float, or air bladder, to support themselves in the water in which they live. Stem may fail, the attached nectccalyces, covering-scales and “tasters" be wanting, but the float always remains with the feeding-polyps, tentacles and sexualbells represented in some form or other. To trace the different modifications in structure among the members of the group, and to show how now one part, and now another is modified, yet leading to no new plan of structure, is a most interesting and

1 Errata in the First Article.-In second line from the bottom of page 631, after the words "17th day of the 2d" add "or 15th," so as to read "17th day of the 2d or 15th month." In third line from the top of page 636, for "governing" read "covering." In second line from the top of page 639, for "each period” read "each two periods."

2 NATURALIST, September, 1880, March, 1881.

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instructive study of these forms of life. Let us, therefore, con

sider in turn the more important genera allied

to Agalma which constitute the so-called Physophorida.

One of the simplest members of the group is a genus in which we have present, as it were the mere skeleton of the Agalma, or simply the float and the stem. To this stem is added feeding polyps and sexual-bells, while all other appendages, as necto-calyces, covering-scales, "tasters" and the like, are wanting. It is, in fact, as if the Agalma had dropped all such as superfluous, and retained only those parts necessary for its life; polyps to eat for the community, a float to support the stem in the water, and sexual organs to reproduce new colonies. We are to consider a genus which is one of the simplest, and on that account can very properly be described in this place.

The name of the animal to which reference is made, is Rhizophysa, which is one of the rare Siphonophores of the Mediterranean and other seas. Its bizarre form and simplicity of structure gives to it an interest second to none of the Physophoride, and as is the case with a study of all aberrant forms, a few words about. its general anatomy may do something to bring about a better understanding of the group of jelly-fishes, of which Agalma may rightly be regarded a representative. The body of Rhizophysa is a simple, flexible, transparent tube, at one end of which is a float (a), Fig. 11, filled with air to support it in the water. This tube, besides being extremely flexible, is highly muscular, and can be contracted into a shapeless snarl under the air bladder, or elongated into a straight, transparent, thread-like axis, as shown in the figure. Sensitive to the least physa. touch of a foreign body, it is seldom quiet, contracting or expanding its length by muscular action of the stem walls. In no

FIG. II. Rhizo

respect does this axis differ from that of Agalma, with which it is morphologically identical.

The appendages to the axis are few in number, but very important. Whatever structures hang from its walls are those only which are necessary to the life of an animal so low in structure as Rhizophysa. There are no swimming-bells for propulsion through the water. It is a passive agent of wind and tide, and like many other pelagic animals, irrespective of itself is helplessly borne along hither and thither as they carry it. Coveringscales also fail along the stem, for they likewise are needless in an organism of this low kind. The organs necessary to the life of the animal, those of digestion and reproduction, cannot fail, and these are all which are to be found appended to the walls of the body of Rhizophysa.

At intervals along the stem, when expanded as shown in Fig. 11, there will be noticed flask-shaped bodies, which closely resemble the polypites of the Agalma colony. These are the "feeding-mouths," and if the distal end of each of these bodies be examined, an opening through which the food is taken in will be found, while in the cavity of the polypite the half digested fragments of small animals betray at once the character of these bodies. The cavity of this polypite communicates with that of the body extending from one end to the other of the axis, through which it is brought into connection with the interior of every other organ of the animal.

From the base of each of these feeding-polyps, there hangs down a long tentacle, beset along its whole length with pendants or tentacular-knobs of a form very different from that of the Agalma. There are three kinds of these pendants, each of which has a characteristic shape which is very different from that figured in my former article, as of the tentacular-knob of Agalma elegans. There is no other structure in the organization of the Siphonophore which varies so much and assumes such a variety of form as the tentacular-knobs, and upon these differences we rely in the main for generic and specific characteristics among the Siphonophores. Rhizophysa has three kinds of these tentacular appendages, and in that respect differs from most other Siphonophores where only one form of pendant is found in the adult.

1 Fig. I was taken from a paper by the author of this sketch in Proc. Bost. Soc. Nat. History, Vol. xx.

Midway between each pair of polypites on the axis of Rhizophysa, there will be noticed a small cluster (e), which when magnified will be found to have a botryoidal shape and to hang from the axis by a small slender pedicel. These organs are ovaries, and correspond with the sexual bells of the Agalma colony, although they never take on a bell shape as is true of the latter genus. How the egg is formed in these clusters, and what the character of the development of Rhizophysa is, no one has yet been able to make out with any degree of certainty.

In recapitulation, these then are the only structures which the skeleton-like Rhizophysa has: an axis (b), with a terminal float (a), polypites, or feeding-polyps, (c), from which arise many tentacles (d), closely set with tentacular knobs, and sexual organs (e) in the form of botryoidal clusters situated midway between each pair of feeding-polyps.

In Rhizophysa we have one of the simplest expressions of the group of animals of which Agalma has been taken as a type. There is but one simpler related animal, and that is a form in which the stem is wholly wanting, and nothing remains to indicate the affinities of the animal with the Physophoridæ except the float. We then have a well-known Siphonophore commonly figured as a representative of the group and called Physalia, or the Portuguese man-of-war.

In this curious animal there is no sign, whatsoever, of stem, swimming-bell or covering-scales, and the float is enormously developed into a bladder, which swims on the surface of the water, and acts in a way as a sail, to the spread of surface in which a raised crest also contributes. The colony of individuals is clustered on its under side, and in that position is borne along through the water. Physalia is, in some respects, the simplest possible form of Physophore and most distantly removed from the type, Agalma. Its close relation to Rhizophysa indicates that it is a true member of the group and not closely related to the floating hydroids Velella and Porpita, which I have already followed McCrady in separating from the Siphonophores.1

The genus Physophora or the "float-bearer," which has given the name of Physophoride to the group, is not perfectly normal, and differs in some respects from the type Agalma. Physophora has never been found in American waters, although quite com

1 Bull. Mus. Comp. Zool., Vol. VI, No. 7.

mon in the Mediterranean, and found likewise in the Atlantic near the Cape Verde islands. It is one of the most beautiful and graceful of all the group to which it belongs.

Physophora differs from Agalma and from all other Physophoridae in this particular, that the polyp-stem to which is affixed polypites, covering-scales, tasters and sexual organs in Agalma, and which takes on the form of a long tube in this animal, becomes reduced in length in Physophora and inflated into a special bag, from the under side of which, in a definite spiral arrangement, structures similar to those of the polyp-stem in the Agalma, hang. It is precisely what would be expected if the portion of the axis of Agalma below the lowest nectocalyx were inflated into a sac, and the appended structures drawn into a spiral line over its under surface. The nectocalyces and the nectostem do not essentially differ in the type and in Physophora. The peculiar tentacular pendants of this animal I will not consider at length, since an account of them would draw me into a description too technical for these papers.

One genus of the Physophoridæ, closely related to the young of Agalma, remains yet to be mentioned. It will be remembered that we described the Agalma as passing through what was called an Athorybia stage. That form is permanently taken by the genus Athorybia, from which it was named. The resemblance of the two is, however, only a likeness in general shape, and is, in particulars, quite remote, for when we study the form of the covering-scales, the tentacular pendants and the fine anatomy of the float, we find very little resemblance between the two. The term "Athorybia stage" is a very convenient one to designate a well marked larval condition of the young even of other genera besides Agalma.

In the genus Athorybia there are no nectocalyces, and if any axis is developed, it is so small as to be practically wanting. In place of swimming-bells, the covering-scales are capable of quite extended motion, and arise directly under the base of the float, thus forming a crown or circlet which encloses that body. Το the outline and arrangement of these structures, as well as the complete absence of nectocalyces, Athorybia owes its peculiar shape. It is probably an arrested embryonic condition resembling closely the young of Agalma, although differing from it in structural details.

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