upon the Homologies of Radiated Animals. Before proceeding to the illustration of their structure in the details which I hope will show that such an homology as our present nomenclature in Zoology would lead to infer, cannot at all be sustained upon any philosophical principles; let me first show that even the general form of the animals belonging to the different great types of the animal kingdom does not agree, and that it is neither correct nor advantageous to the progress of science to use similar expressions to designate their various parts as has been usual among naturalists, ever since comparative anatomy aimed at illustrating the peculiar structure in all the classes of the animal kingdom. Radiata, are animals whose prominent character rests in the peculiar outline of their body, as well as the peculiar relative position of their parts. Whatever may be the modifications which they present in different classes and families, they can all be reduced to a typical form, best expressed by a star, or a circle, with an internal radiated arrangement of the parts. And all animals, whose structure cannot be reduced to such a diagram, must be removed from this great group.

That intestinal worms do not truly belong to the type of Radiata, as Cuvier admitted, has been satisfactorily shown, by the investigations of all those naturalists, who have studied the subject more recently. That Infusoria again, do not constitute a natural group, is best exemplified by a comparison of the different families which have been combined under that name.

The Rotifera, for instance, can plainly be shown to belong to the class of Crustacea, as the closest homology can be traced between them and the Malacostraca. I have recently shown that a great many of the Polygastrica are simply larval forms of different worms, especially those of the type of Paramacea; whilst others, such as the Vorticella, truly belong to the Bryozoa, and are therefore true Mollusca, and a great many others, especially the Polygastrica anentera, are truly vegetables.

But even after removing these classes from among Radiata, we find several groups left in the class of Polypi, which are neither true Polypi nor really Radiata. Such are, for instance, Foraminifera, which I have recently ascertained to be the strongest resemblance to the embryonic forms of Gasteropoda, and to be therefore a low form of gasteropodian mollusks, whilst the Bryozoa constitute a natural group, closely allied to the tunicated mollusks, and therefore belong to the class of Acephala.

Again, the so-called Hydroid Polypi, do not in my opinion, belong to the class of the Polypi, but should constitute a special family among Medusa, to which they bear the same relations as the Crinoides with their stems, bear to the free-moving starfishes. But in order to show that these views are strictly correct, it is necessary for me to illustrate first the homologies of Radiata in general, as well as the organic characters of their different classes. While it will be obvious that the presence or absence of a stem is no more a peculiar character of Polypi, than of any other classes, as we have free Polypi among the Actiniæ and fixed Echinoderms among the Crinoids, and there is no reason why there should not be also fixed Medusa, provided their true structure agrees more closely with that of the true free Medusa, than with that of Polypi.

The relation of Hydroid Polypi to Medusa has already been alluded to by me in my lectures on Comparative Embryology, but I hope here to show more fully that this view is sustained by the closest investigation of their special homologies.

Echinoderms, Medusa, and Polypi, restricted as above mentioned, constitute, in my opinion, the only true representatives of the type of Radiata, and form, in this limited connection, a most natural group in the animal kingdom, which agree entirely in their radiated structure, and differ only in the manner in which the structure is developed in the different classes.

In Polypi, the body constitutes a simple sack with internal radiated partitions and a central aperture at one extremity surrounded by fringes and communicating with an inner sack which itself empties into the main cavity of the body.

In Medusa, the main cavity of the body is not sub-divided, but limited by a large development of the substance of thin wall, and from this central cavity arises a system of tubes which divide in a radiated arrangement into that substance. The mass itself projects more or less in the form of an umbrella or of lobes with various fringes around their margin; whilst the mouth projects from the centre surrounded by various loose appendages, or in the form of a proboscis arising from a more intimate connection of the same parts which in others constitute the appendages of the mouth.

Compared with the Polypi, therefore, the Medusa present this peculiar difference, that their mouth is prominent, more or less tubular, and provided with a peculiar fringed apparatus, whilst in Polypi it is in one level with the walls of the body, and simply folded at its open

ing. So that, if any comparison can be instituted between them, we shall only trace an homology between those folds and the appendages of the mouth in Medusa, and not between those and the tentacles of Polypi. On the contrary, the tentacles of the Polypi are homologous to the fringes, or various appendages which hang from the margin of the disk of true Medusa. On the other hand, in Polypi the internal cavity is divided into two distinct spaces, one circumscribed with peculiar walls, and forming a kind of stomach, and the other below formed simply by the external walls of the animal itself, from which arise partitions which sub-divide partly this lower cavity into open cells, communicating above with the cavity of the tentacles, which correspond in number to the number of the partitions.

In the Medusa, on the contrary, the main mass of the body is solid, nd traversed by a regular system of radiating tubes, communicating at the periphery with a circular tube extending all round the margin of their disk. And the cavity of the stomach communicates with this system of tubes by a few openings, and not by a single large central one as in Polypi. Notwithstanding the fundamental difference which this arrangement introduces between these two classes, it cannot be overlooked that the system of tubes of Medusa is homologous to the partitions of Polypi, but constitute a lower condition of a similar apparatus, as in Medusa the tubes are regularly ramified, and again. combined by a circular canal, whilst in Polypi all the partitions communicate freely with each other in a central general cavity, and open freely into the tentacles, and even through the tentacles with the surrounding medium.

Moreover the sexual apparatus of Polypi consists in bunches of eggs, or in spermatic chords attached below the stomach to the free margins at the partitions in the main cavity of the body, whilst in Medusa, especially in those which resemble nearest the Hydroid Polypi the sexual apparatus forms bunches of eggs or of spermaria between the outer circle of tentacles and the fringes surrounding the mouth, sometimes more closely connected with the disk itself,- sometimes adhering with the outer surface of the proboscis. And now in Hydroid Polypi, especially in Tubulariæ, the bunches of eggs hang from the base of the external surface of the proboscis within the circle of tentacles which surround the main mass of the body; an homology which establishes the closest relation between true Medusa and Hydroid Polypi. Indeed, it can now be said that the so-called Hydroid Polypi are true Medusa, resting upon strings, and not

merely polyp-like animals producing alternate generations of various free Medusa.

The intimate connection which exists between Hydroid Polypi and Medusa, which has been so beautifully established by the investigations of Sars, Steenstrup, Liebold, Loren, Du Jardin, Sir John Dalyell, and others, might have been ascertained long ago, merely upon homological evidence, if the fact of their resting upon a stem had not constantly misled observers to consider them as Polypi.

The conclusion of these remarks allows me therefore to state, most decidedly, that Hydroid Polypi are true Medusa, resting upon stems. The homologies thus traced have led to some other discoveries of minor importance.

Thus, the position of the eye specks around the margin of true Medusa between their tentacles, pointed to the place where similar organs should be searched for among other Radiata. They were accordingly discovered between the tentacles of Lucernaria, in a position identical with that which they maintain in Aurelia. This fact will show once more how many material additions to our knowledge of the structure in the details of animals, may be obtained by tracing their more general relations. It is somewhat more difficult to trace the homologies between Echinoderms, and Medusa and Polypi, as the first of these classes is so much more complicated in its structure than the two others. However if we consider only the general relations of their organs, we shall experience no difficulty in comparing them.

In the first place, the external wall of the body in Echinoderms assumes a highly complicated structure, by the peculiar development of isolated calcareous plates, arranged in a most admirable symmetrical order, and affording by their combinations a solid support, and points of attachment for most of their organs.

But however complicated these walls may be, and I need only refer to the works on Comparative Anatomy for the details, it is obvious that these walls which enclose the general cavity of the body correspond to the gelatinous mass of Medusa and to the membranous walls of Polypi with their solid parts. The mouth, which is soft in the other classes, is here frequently surrounded by solid parts and even by a complicated system of jaws and teeth, surrounded by simple fleshy lip.

The isolated fringes on the margin of Polypi and Medusa, are transformed here into complicated gills, protruded between the solid covering, and the membranous covering of the mouth; and these gills are truly homologous to the tentacles of Polypi and Medusa;

we need only remember the fringed tentacles of some of the Actinia to be satisfied of their identity. The inner gills with their ambulacral pedicelli are foreshadowed in Polypi in the minute vertical rows of pores, which are observed on the outer surface of the Actinia, between the partitions. Similar openings reappear, nevertheless, in Echinoderms, in the form of a peculiar system of waterpores. The greatest progress in the complication of structure in this class, however, is shown in the development of the digestive system and its appendages, which constitutes here a regular alimentary canal, with various divisions generally accompanied with a complicated system of bloodvessels. The sexual organs also grow more independent, constituting a distinct system with special openings, and indeed the prominent structure of Echinoderms may be said to consist in the localization of the different apparatus upon some few of their rays, whilst there are as many radiating structural appendages as there are rays in the lower types.

In Polypi the number of tentacles is sometimes very great. In Echinoderms there are only five bunches of gill-like fringes. The ovaries are nearly as numerous in Polypi as the inner partitions; in Echinoderms there are only five, sometimes only four, and even only two ovaries. In Polypi the cavity in which the nutritive fluid moves is sub-divided by partitions into many open cells. In Echinoderms the homologous system constitutes a cylindrical simple tube, or sack, which branches only in the lower forms of that class, as in the starfishes, where numerous liver-like appendages are also attached.

The circulating system in Polypi is directly connected with the alimentary sack, whilst in Medusa it is more closed in itself, especially in the Ctenophora, and in Echinoderms the distinction is carried out still further.

The nervous system where it is well developed in these animals, forms a ring around the entrance of the alimentary canal with radiating threads to the different rays terminating at the eye specks.

Among Medusa, the nervous ring presents a similar arrangement, though this system is less distinct in its substance, a circumstance which leads me to suppose that our efforts to discover a peculiar nervous system in Polypi may be altogether vain, as it is possible that their sensitive faculties are concentrated in these animals in a particular set of cells without peculiar distinct connection. From the compari

* See my paper in Comptes-rendus, 1847.