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THE POLYZOA RECOGNISED AS A DISTINCT GROUP.
with scientific men, Thompson was quite unaware, when he published the result of his researches, that Grant and Edwards had been before him in the field; his observations are, therefore, original and independent, and, as he tells us they were made in the summer of 1820, it would seem that it was only the delay of publication that has deprived him of the honour of being the first to record a discovery so important in the history of zoology.
In 1834 Ehrenberg published his 'Memoir on the Corals of the Red Sea.'* In this work he proposed a new classification of the so-called polypes, dividing the entire group into two great sections-the Anthozoa and the Bryozoa; the former embraced the true radiate forms, the latter corresponded to the Polyzoa of Thompson. Though the term Bryozoa had already been used by him in a number of the 'Symbolæ Physicæ,' published in June, 1831, the priority of publication is still left with Thompson's name, and though Ehrenberg's term is in general use upon the Continent, and is largely adopted even by English writers, simple justice and the laws of natural-history nomenclature demand the adoption of the term Polyzoa, and it is it, therefore, which I have employed in the present Memoir.†
Notwithstanding, however, the completeness with which the ascidian type of structure had now been recognised in the Polyzoa, naturalists had not yet emancipated themselves from the old notion that the closest affinities of these animals were still with the Polypes, and the Polyzoa, therefore, long continued to be classed with the Polypes, of which they were still considered as a group, though with distinct peculiarities, through which the Polypes manifested an affinity with the Tunicata.
It is not easy to say to whom we are indebted for the first absolute withdrawal of the Polyzoa from the Radiate sub-kingdom, and their location among the Mollusca. The obvious justice of the step must have simultaneously presented itself to every naturalist who had made the matter a special subject of study, while the important division of the Molluscan subkingdom by Milne Edwards into the two primary sections of the Mollusca and the Molluscoida, the latter including the Tunicata and the Polyzoa, leaves nothing now to be desired in the systematic location of the Polyzoa.
In the history of progressive discovery which has thus been sketched, eight distinct epochs must be noted, each characterised by some one step which has more or less directly led to the views at present entertained of the true affinities and systematic position of the Polyzoa. 1. The assertion by Imperato of the animality of coral. 2. The discovery by Marsigli of the polypes of coral, which he mistook for its flowers. 3. The determination of the true nature of these polypes by Peysonelle. 4. The discovery of the Hydra by Leuwenhoeck. 5. The discovery of the "Polype à Panache," and the determination of its structure by Trembley and Baker. 6. The determination of the structure of certain marine Polyzoa by the independent and nearly simultaneous labours of Grant, Edwards, and Thompson; and the recognition of the affinity of these productions with the compound Ascidians by Edwards and Thompson. 7. The designation, by a common independent name, of these animals by Thompson. 8. The entire withdrawal of the Polyzoa from the Radiata and their association with the Mollusca.
* 'Beiträge zur physiologischen kenntniss der Corallen-thiere im algemeinen, und besonders des Rothen Meeres.'
† See an admirable criticism by Busk on the Priority of the term Polyzoa, in the 'Annals of Nat. Hist.,' vol. x, 1852.
Fig. 1. Exserted.
Fig. 2. Retracted.
a. Region of mouth, surrounded by tentacles. b. Alimentary canal. c. Anus. d. Nervous ganglion. e. Membranous investing sac. f. Testis. f'. Ovary. g. Retractor muscle.
Let us imagine an alimentary canal, consisting of œsophagus, 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
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
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 Canacium.† The cœnocium 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 cœnocium 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.
*Πολυποὺς, εἶδος. † Κοινὸς, οἰκίον. † Εκτος, κύστις.
§ ̓́Ενδον, κύστις. || Λόφος, φορέω. ** Περὶ, γαστὴρ.
Determination of Aspects.
Another important point, which should be settled at the very outset of our anatomical inquiries, is the exact sense in which we are to use the terms employed to indicate the different aspects of a Polyzoon. This is the more necessary as the terms used for this purpose, in the lescription of the invertebrate animals generally, are frequently employed in the vaguest possible way, the same term being often applied by different authors to very different aspects of the animal.
In fixing the meaning of the terms anterior and posterior we may assume the position of the mouth as indicating the region of the animal which is to be designated as anterior, while the posterior region will then be that diametrically opposite.
In fixing the dorsal and ventral regions greater difficulty is met with. Mr. Huxley, in his very ingenious and philosophic Memoirs on the Homologies of the Mollusca,* rejects the terms dorsal and ventral altogether; generalising the Molluscan form under the conception of an ideal archetype, and finding the heart occupying one side, and the great nervous centres placed upon the opposite, he gives to the former region the name of "hæmal," and to the latter that of "neural," thus applying to the Mollusca the terms already so happily employed by Owen in his designation of the regions of the vertebrate skeleton.
These terms have the advantage of stating a simple fact, and of thus avoiding the ambiguity which so often attaches to the terms dorsal and ventral. I shall, therefore, willingly adopt them in the present Memoir, and notwithstanding an apparent contradiction in designating as "hæmal" any portion of an animal totally deprived of a blood-vascular system, I shall call that region of a Polyzoon on which the nervous ganglion lies the "neural," and the opposite region, that, namely, which corresponds to the part of an Ascidian which contains the heart, the "hæmal."
Tabular view of the Orders and Sub-orders of Polyzoa.
The reader will be further assisted in the anatomical inquiry in which we are now about to be engaged, by having placed before him here the following scheme of the orders and sub-orders under which all the species of Polyzoa, both marine and fresh-water, admit of being arranged:
*English Cyclopædia,' 1855, article "Mollusca ;" and Phil. Trans.,' 1853.
* Phylactolæmata (from puλáoow, to guard, and λaqua, 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.
+ Gymnolæmata (from yvuvos, naked, and λačua, 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 Gymnolæmatous 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 Gymnolæmatous Polyzoa. (Voyage of "The Rattlesnake," vol. i, Appendix, p. 346.)