together in modern systems of zoophytology under the name of Hydrozoa*' or Anthozoa Hydroideat.

These also must propagate by ova, in order that the species may be dispersed. In some species the germ-cells are metamorphosed into ova at particular parts, and the concomitant growth of the soft tissue and outer crust furnishes those ova with a capsule (Pl. I. fig. 2, ƒ); which modification in the growth of the coralline Prof. E. Forbes compares with that "metamorphosis in flowering plants in which the floral bud is constituted through the contraction of the axis and the whorling of the individuals borne on that axis, and by their transformation into the several parts of the flower."

The ova may escape from the ovi-capsule in the condition of ciliated locomotive bodies, called 'planulæ' by Sir John Graham Dalyell; or the planulæ may be hatched in the interior of a polype-individual developed from the summit of the ovi-capsule, and which, after liberating them, may wither and fall like the flower of a plant, as in some Campanulariæ ‡. Or a generative individual of a particular form § may be developed and become detached, and, by its own power of locomotion, carry the contained ova to a distance from the composite and fixed-group of nutritive individuals.

The ova may be developed within the bell-shaped Acalephoid prior to its detachment, as in the Coryne vulgaris, observed by Wagner ||, or not until it has become de

* Owen, Lectures on the Comp. Anat. and Physiology of the Invertebrata, p. 82, 8vo, 1843.

↑ Johnston, History of British Zoophytes, p. 5, 8vo, 1846.

Lister, Phil. Trans. 1834, p. 375, pl. 10. fig. 1, 5, 6. (See Pl. I. fig. 2, g, i.)

§ E. g. the Medusa octocilia and duodocilia of Dalyell, from Eudendrium ramosum ; and the Tintinnabulum or Bell-medusa (Pl. I. fig. 2, 1) observed by the same author to be developed from the Campanularia dichotoma.—Edinb. New Philosophical Journal, vol. xxi. 1836, p. 91. Isis, 1833, p. 256, t. xi.

tached and acquired the full characters of a bare-eyed Medusa*

This remarkable phænomenon is best shown in the claviform Corallines, and has been especially described by Lovent in an excellent memoir on the Syncoryne ramosa, and by Steenstrup in the Coryne Fritillariat. This species originally developes a many-armed digestive polype or individual, which retains a large proportion of unchanged germ-cells: these, by the stimulus of the excess of nutriment, begin to repeat the process, and push out buds in an analogous position to that in the Hydra fusca, viz. around the base of the stomach of the first or parent animal: but the buds, instead of repeating the form and condition of that animal, take on a higher form, resembling that of a bell-shaped Medusa; they become detached and swim off to a distance, forming and discharging the ova, which, as Steenstrup conjectures, in their turn develope the fixed polype-shaped Coryne.

This stage of the cycle has not yet been the subject of observation; but, by the analogy of the larger Medusa, is the more probable process than that direct metamorphosis of the medusiform individual into the pedunculate polypoid individual, which V. Beneden has described by the aid of a conjectural figure in the Tubularia §.

The medusiform ovigerous locomotive or distributive individual of the Coryne and Campanularia dichotoma is obviously homologous with the polypiform ovigerous individual, which seems to nursc, as it were, the ova into 'planulæ' in the Campanularia geniculata; and the nutritive gemmiparous polypiform individuals in all the com

* See the beautiful and philosophically treated monograph on the Gymnophthalmata, by Prof. Edw. Forbes ; published by the Ray Society. 4to, 1848.

+ Wiegmann's Archiv für Naturgeschichte, 1837, p. 322, t. vi. 'On the Alternation of Generations,' translation by Ray Society, 8vo, 1845, p. 26, pl. 1. figs. 41–45.

Recherches sur l'embryogénie des Tubulaires, 4to, 1844, pl. 2. fig. 5.

pound Radiaries would seem, rather than the oviparous medusiform ones, to manifest the typical form of the species; as the leaf is a more typical form of the plant than the parts of the flower; and as the free-swimming Cirripedal larva, with its pedunculated eyes, is more typical of its class than is the blind and fettered multivalve Barnacle into which it is so marvellously transformed. Superadd, however, distinct nutritive and circulating organs to the free-moving ovigerous individual from the rooted polype, and prolong its existence, and it would then cease to have the ancillary character of a nurse to the ova of the fixed individuals, and would assume that of the perfected form of the species, and such in fact is the case with the larger gelatinous Radiaries called Medusa.

The egg of the species called Cyanea aurita is devoid of a chorion*, and consists, after impregnation, of the germmass only, formed according to the before-defined process for the dissemination of the spermatic principle t. By the combination and metamorphosis of the peripheral series of secondary germ-cells a ciliated epithelium is formed ‡: an assimilative cavity § results from the liquefaction of certain · central germ-cells: the germ-mass grows, elongates, and is

* Siebold, Beiträge zur Naturgeschichte der Wirbellosen Thiere, 4to, 1839, p. 21.

+ The external signs of these universal preliminary steps in development ab ovo are figured in the above-cited work, in tab. 1. figs. 1–13. The preliminary internal changes may be inferred from what was observed by Siebold and Bagge in the ova of the Strongylus auricularis (De Evolutione Strongyli, 4to, 1841), some of whose figures are copied in my 'Lectures on the Invertebrata,' 8vo, 1843, p. 77, figs. 33-44, where I remark, as Dr. Martin Barry had, also, independently remarked (see the note which he communicated to the same work) :-"This preliminary division of the clear central cell to the spontaneous fission of the yelk is closely analogous to that division of the central cell in the polygastrian animalcule, preparatory to the spontaneous division of its body into two individuals, which Ehrenberg described.” (See Pl. I. figs. 4-13 of the present 'Discourse.”)

↑ Siebold, loc. cit. fig. 19.

Ib. fig. 15.

thus metamorphosed into an animal, which, like the 'planula' of the Hydrozoa, typifies the ciliated infusorial Leucophrys; ciliated lobes, like those of a Rotifer, are next developed from one extremity; these grow into arms, and the whole animal assumes the form of a polype: as such it was originally described by Sir J. G. Dalyell, under the name of Hydra Tuba, and in this state it propagates by gemmation.

The condition of this Parthenogenesis,' or power of propagation by the virgin larval polype, appears to reside, as in the Hydra fusca, in the following structure: "The body and tentacula of the larva are composed of two distinct layers, an internal and external. The internal layer chiefly consists of nuclei and nucleated cells of various sizes, some of them containing a large number of nuclei; while the external is chiefly composed of a structureless substance with numerous minute nuclei disseminated through it." Sir J. G. Dalyell obtained a colony of eighty-three Hydra Tubæ, by buds, from one parent. But the procreative force is not hereby exhausted. After one, two or more years of captivity the body of the polype has been observed to become thickened and impressed by circular grooves: these, deepening, divide the outer surface into rings, and this annulated part of the body may be sup ported on a contracted and smooth base; the whole polype being lengthened by the successive development of the segments so indicated: the margins of these segments next shoot out short tentacles in eight pairs, and then the whole of the annular part† is resolved by a series of spontaneous fissions into as many small medusiform discs, which by growth and a minor amount of metamorphosis are developed into true Medusæ. In certain of these individuals sperm-sacs

Dr. J. Reid's 'Observations on the Development of the Medusæ.'— Taylor's Annals of Natural History, 1848, p. 26.

↑ The base may remain, reproduce the arms of the Hydra, and again propagate by gemmation and spontaneous fission.

and spermatozoa are developed; in others, ovisacs and ova: thus distinct males and females are produced, combining the nutritive with the generative functions. The ova are impregnated, are sheltered for a time in special marsupial pouches, the parent also performing the part of a nurse, and then the young brood issues forth under its ciliated or infusorial form. From the polygastric type it passes into the rotiferous one, from this into the polype type, and all the individuals propagated by gemmation under the latter form are ultimately resolved by spontaneous fission into the male or female oviparous Medusa.

I will cite a few other instances in which a species is represented by a series of individuals of different powers and forms succeeding each other in a cycle. Certain freshwater snails arc infested by Entozoa of the order Trematoda or fluke-worms; as the Limnæa stagnalis e. g. by the Distoma tarda. The ova, or products of the ova, of this species are found, in early summer, adhering in vast numbers to the inner surface of the respiratory cavity and to the exterior of the lobes of the liver and generative organs of the snail; where they increase in size, and detach themselves as free animalcules, assuming a bright yellow colour, whence they were called by Bojanus "konigsgelben Würmern," and manifesting a twisting vermicular motion. If one of these be microscopically examined, none of the lineaments of the organs of the future Distoma can be discerned; they resemble in structure rather the Gregarinæ, consisting in fact of little else than the germ-mass, a small proportion of which may have been metamorphosed to form the smooth outer skin.

As the growth of this Gregariniform parasite proceeds, a progeny is seen to rise in its interior by the aggregation round separate pellucid centres of the nuclear matter into numerous independent germ-masses, and their development into embryos: these are first manifested by the appearance of an anterior and a median circular sucker, and then they acquire cephalic spicula and a