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Plan Of A Polyzoon.
Fig. 1. Exserted. Fig. 2. Retracted.

a. Region of mouth, surrounded by tentacles, b. Alimentary canal, c. Anus. d. Nervous ganglion.
<?. Membranous investing sac. /. Testis. /'. Ovary, g. Retractor muscle.

Let us imagine an alimentary canal, consisting of oesophagus, stomach, and intestine, to be furnished at its origin with long ciliated tentacula, and to have a single nervous ganglion situated on one side of the oesophagus. Let us now suppose this canal to be bent back upon itself towards the side of the ganglion so as to approximate the termination to the origin. Further, let us imagine the digestive tube thus constituted to be suspended in a fluid contained in a membranous sac with two openings, one for the mouth and the other for the vent; the tentacula alone being external to the sac. Let us still further suppose the alimentary tube, by means of a system of muscles, to admit of being retracted or protruded according to the will of the animal, the retraction being accompanied by an invagination of the sac so as partially or entirely to include the oral tentacula within it; and if to these characters we add the presence of true sexual organs in the form of ovary and testis occupying some portion of the interior of the sac, and the negative character of the absence of all vestige of a heart, we shall have perhaps as correct an idea — apart from all theoretical considerations of homology or derivation from an archetype—as can be conveyed of the essential structure of a Polyzoon in its simplest and most generalised condition.

To give, however, more actuality to our ideal Polyzoon, we may bear in mind that the immediately investing sac has the power, in almost every case, of secreting from its external

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8 DEFINITION OF TERMS.

surface a secondary investment of very various constitution in the different groups; and we may, moreover, conceive of the entire animal with its digestive tube, tentacula, ganglion, muscles, generative organs, circumambient fluid, and investing sacs, repeating itself by gemmation, and thus producing one or more precisely similar systems holding a definite position relatively to one another, while all continue organically united, and we shall then have the actual condition presented by the Polyzoa in their fully developed state.

Definition of Terms.

The old notion, which, by mistaking the zoological rank of the Polyzoa, erroneously referred them to the class of the Polypes, caused the same terms to be applied to them which were also used to designate the various parts of the true Polypes. The recognition, however, of a type of structure in the Polyzoa totally distinct from that of the Polypes proper, necessitates a change in the terminology employed in their description. On these grounds I have ventured to substitute some new terms for those previously used, while our increased knowledge of polyzoal structure necessitates the use of certain additional terms of which we have no representatives in the descriptive terminology of previous authors. For the term Polype, therefore, originally applied not only to the polypoid Radiata, to which its use ought to be confined, but also to the retractile portion of the Polyzoa, I have substituted in the following memoir that of Polypide* To the common dermal system of a colony, which, as well as the solid basis of the true polypes, was formerly known under the names of Polypary and Polypidome, I have applied the term Ccencecium.^ The coenoecium is composed almost universally of two perfectly distinct tunics; to the external I have given the name of Ectocyst,\ and to the internal that of Endocyst.^ The sort of disc or stage which surrounds the mouth and bears the tentacula, I have called Lophophore.\ The Epistome% is a peculiar valve-like organ which arches over the mouth in most of the fresh-water genera. The Perigastric** space is the space included between the walls of the endocyst and the alimentary canal.

The terms now enumerated are such as I believe the nature of the subject strictly requires. I employed most of them in my 'Report on Fresh-water Polyzoa' published in 1850, and though I am fully aware that the changing of an established terminology is highly objectionable where it can possibly be avoided, yet in the present case, where new facts have been accumulated requiring new words for their expression, and where the very same terms have been in two different classes of animals loosely applied to organs in no respect homologous, the purposes of a rigidly scientific description can, I believe, only be served by some such change as that suggested.

Besides these terms, and some which will be explained as they occur, two others in common use ought to be here defined. The cells are the little chambers of which the coenoecium is made up, and in each of which a polypide is lodged. The part of the cell through which the polypide admits of protrusion and retraction is the orifice of the cell.

*rio/\u7rovc, fISoc. t Koivoc, oifc/of. + 'E/oroc, Kvanq. § "RvBov, Kvtrris. || Ao^ioc, <p»piw. *[ Etti, OTOfta. ** I Inn, yaart\p.

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* Phylactoltemata (from <f>v\aaow, to guard, and Xiu/na, the gullet, in allusion to the epistome placed at the entrance of the alimentary canal) corresponds in part with the Hippocrepia of Gervais. The Hippocrepia of the French zoologist, however, constitute in reality an artificial group. Being essentially characterised by the possession of a crescentic lophophore, they necessarily exclude not only Pedicellina, but even Fredericella, whose relations with the species furnished with a crescentic lophophore are of the most intimate kind. Hippocrepianism, therefore, though of great interest as a morphological fact, tending, as will be afterwards shown, to throw much light on certain homological questions, cannot be employed as the determining character of groups more comprehensive than those of generic rank.

t Gymnolamata (from -yi^uvoe, naked, and Aaj/ua, in allusion to the absence of an epistome) corresponds to part of the Infundibulata of Gervais.

% For the structure of Pedicellina, see Note, p. 19.

§ The location of Urnatella among the Gymnolaematous Polyzoa must for the present be viewed as a provisional expedient, subject to alteration as its structure becomes better known. See the description of the genus farther on.

| The terms Cyclostomata, Ctenostomata, and Cheilostomata, were proposed by Busk, to indicate the primary subdivisions of the marine Gymnolaematous Polyzoa. (Voyage of "The Rattlesnake," vol. i, Appendix, p. 346.)

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