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view a number of important facts, and has defended it with more ingenuity than it has received from any other anatomist.*
Imagining an ideal molluscan archetype, Mr. Huxley derives from this general plan all the special modifications presented by the molluscan sub-kingdom. From this archetype he thus derives the polyzoal form. “Conceive the abdomen of the archetype to be greatly prolonged, the neural region with its appendages, the organs of sense, and the heart, remaining undeveloped, so that the anus comes into close apposition with the oral extremity, while the edges of the latter are produced into long ciliated tentacles; and the result will be a Polyzoon, which needs only the power of gemmation to give rise to those composite aggregations which are so characteristic of the group.”+
In these words we have an excellent and easily comprehensible enunciation of the essential morphological characters of a Polyzoon.
In deriving the ascidian form from the archetype, “it is to be remarked,” he says, the first place, that the pharynx, large in the Polyzoa, becomes comparatively enormous in the Ascidians; while the tentacles, which were very large in the Polyzoa, are in the Ascidians comparatively small. Next with the development of a post-abdomen, the intestine acquires a hæmal flexure; but instead of the anal aperture remaining on the hæmal side, it is bent round by the same process as in Spirialis and Limacina, but in the inverse direction. Suppose, with all this, that a mantle has been developed, and that its free margin, remaining small and narrow, has followed the anus to the neural side, while its cavity has extended up on each side of the pharynx to the middle line of the hæmal surface of the latter, carrying to a great extent a process of which the outline may be seen in Cymbulia, and giving rise to the atrium ; -imagine also that the sac thus constituted externally by the inner surface of the mantle (third tunic) and internally by the pharynx, becomes perforated by minute apertures—and the result would be an Ascidian.”I
Now, if the above be an exact statement of the true morphology of the Ascidian, it is inconsistent with the views attempted to be demonstrated in the present paper. After a very careful consideration, however, I have not been able to adopt it. Its essential feature consists in the peculiar explanation it offers of the formation of the “atrium," the term given by Mr. Huxley to the space, including the cloaca, which intervenes between the branchial sac and the third tunic of the Ascidiæ; and it necessarily involves the supposition that the walls of this cavity wrap themselves round the pharynx in the manner of a serous sac, just as the serous lining of the pericardium in the higher animals wraps itself round the heart; and that the pharynx is therefore, properly speaking, external to the cavity of the atrium. I have never succeeded in satisfying myself of the reality of this condition, and must still believe that the walls of the atrium simply surround the branchial sac, without being reflected on its sides; that the branchial sac is, therefore, properly within the cavity of the atrium, just as the mouth and labial palps are within the mantle-cavity of the lamellibranchiate Mollusca, and the tentacular crown of the Polyzoa within the tentacular sheath during the retracted state of the polypide.
* • British Association Reports' for 1852, and 'English Cyclopædia,' 1855, article “Mollusca."
From what has now been said, it will perhaps be admitted that the only homological view deducible from the comparison which we have endeavoured to institute between the two great sections of the Molluscoida, is that insisted on in the present memoir. But no view of the homological relations between two sets of organs can be accepted, unless it can also be shown that the conclusions derived from anatomy are confirmed by the history of development, or at least unless it appears that this history presents nothing irreconcileable with the conclusions which rest on anatomical evidence alone.
Unfortunately, the difficulties which beset this part of the subject render us still deficient in the class of facts here needed. We owe to Krohn* some of the most recent investigations into the development of the Ascidiæ, and, so far as these go, I believe them to be in accordance with the views advanced in the present memoir. In the account given by Krohn, some observations are still wanting before we can determine the exact nature and sequence of all the steps in the development of the Ascidian embryo. From this account, however, it appears that a cavity, which represents that of the future respiratory sac, shows itself at a very early period in the interior of the embryo, having immediately behind it the first indication of the alimentary canal as a tube of uniform diameter bent into a loop. The walls of this cavity are at first in close apposition with those of the thoracic chamber, which completely encloses it, so that the two cavities at this stage appear as one, being exactly co-extensive; we soon perceive, however, that the proper branchial sac becomes perforated by four apertures, the first appearance of the branchial stigmata, and at the same time becomes detached, in the region of these apertures, from the walls of the thoracic chamber, which had previously itself been perforated by the respiratory and cloacal orifices (the latter at first double, but afterwards single by coalescence). The branchial stigmata subsequently increase in number, the branchial sac at the same time detaching itself more and more from the walls of the thoracic chamber, while the space thus produced, and bounded internally by the walls of the branchial sac and externally by those of the thoracic chamber, becomes the cloacal chamber of the adult. In the intervals of the branchial stigmata, the transverse and longitudinal respiratory bars make their appearance, but the mode in which they originate is not evident; nor have we any account of the mode of formation of the branchial or thoracic sinus, the first indication of their existence being the current of blood which is seen to flow through them after the heart has begun to act. The alimentary canal, as has just been said, had already shown itself. It has now become more developed, and presents a division into the three regions of csophagus, stomach, and intestine; while the mouth opens into the bottom of the branchial sac, and the opposite extremity is found in the cloacal chamber, or space formed by the separation from one another of the walls of the branchial sac and thoracic chamber.
We have here, then, a series of facts which admit of an easy comparison, so far as they go, with the corresponding steps in the development of the Polyzoa. It will be recollected that, in the development of the bud of a Polyzoon, a minute anterior cavity, closed on all sides, is the first evidence of differentiation. From the walls of this cavity the lophophore subsequently detaches itself, and then gives rise to the tentacles which gradually develop themselves from its margins. The cavity is here to become that of the tentacular sheath,
Krohn in Müller's Archiv,' 1852-3; translated in the new series of Taylor's 'Scientific Memoirs,' August, 1853.
which we have assumed as homologous with the thoracic chamber of the Ascidian ; and the lophophore, which represents the thoracic sinus of the branchial sac of the Ascidian, becomes separated from it just as the branchial sac separates itself from the walls of the thoracic chamber of the Ascidian.
The development of Salpa has been especially studied by Leuckart. From the greater simplicity of the respiratory system in the Salpæ, it is easier to compare the Salpæ and Polyzoa with regard to the development of this system, than it is to institute a similar comparison between the Ascidiæ and the Polyzoa. Now, the researches of Leuckart fully bear out the views here taken. As in the Ascidians so also here does the first trace of special organs show itself in the formation of an internal closed cavity, the primary sketch of the branchial chamber. From the walls of this cavity the rod-like gill is then separated, leaving behind it a space which is to become the cloaca. The alimentary canal, about the same time, makes its appearance; and it is not until a comparatively late stage of the development that the branchial chamber communicates with the exterior by the branchial and cloacal orifices.
It is plain that, in its essential points, the resemblance here with what we have seen to occur in the Polyzoa is complete. The closed cavity, constituting the early stage of the branchial chamber in the Salpa, resembles even in minute particulars the early stage of the cavity of the tentacular sheath in the Polyzoa, as will be at once evident from an inspection of Leuckart's figures, while the rod-like gill is developed in the interior of the branchial chamber, in all essential points just as the lophophore of the Polyzoa is developed within the cavity which is ultimately to become that of the tentacular sheath in these animals. In the Polyzoa the development is continued by the shooting out from the lophophore of hollow
processes which are to become the tentacula. In the Salpa these processes are never developed, and the gill continues in the condition of a polyzoal lophophore, with the tentacula suppressed.
It appears to me, then, that the points of connection attempted to be established in the present memoir between the Polyzoa and the Tunicata are those which really exist in nature; but there still remain to be considered some other relations of great interest presented by the Polyzoa. We know that the affinities of organized beings are not confined to those which directly unite them to some single allied form. On the contrary, they stream forth from them in various directions, and bring into a community of plan many different types.
Besides the close affinity between the Polyzoa and the Tunicata, another series of homologies can be pointed out, bringing the Polyzoa into intimate relation with the Brachiopoda. Mr. Hancockt had maintained the resemblance between the arms of the lophophore in the hippocrepian Polyzoa and the oral arms of the Brachiopoda, a resemblance singularly striking, and pointing to an important homological relation. In confirmation of this view, he has further called attention to the resemblance between the parieto-vaginal muscles of the Polyzoa and the pedicle muscles of Terebratula, and has compared their action, in one case in closing the orifice of the cell, in the other in closing the valves of the shell.
The resemblance between the two sets of muscles is certainly obvious, and may possibly
* Zoologische Untersuchungen, Zur Anatomie und Entwickelung der Tunicata,' 1854.
+ On the Anatomy of the fresh-water Bryozoa, by Albany Hancock, Esq. "Annals and Mag. of Nat. Hist.,' 1850.
indicate homological identity, though we should be cautious in concluding as to homological relations from mere resemblance; where functions of the same kind are to be performed, there must necessarily be a certain degree of resemblance in the instruments, and this resemblance may show itself not only in form but in position and relation to surrounding parts, though the organs themselves be fundamentally different; and it needs the light afforded by development before our conclusions as to homological identity can be considered as fully established, while our ignorance of facts connected with the embryology of the Brachiopoda deprives us of this important test in the present instance,
Mr. Huxley* has thrown still further light on the relations between the Polyzoa and Brachiopoda. He compares the relative positions and mode of articulation of the valve which closes the cell in the marine cheilostomatous Polyzoa and of the peculiar bodies known as avicularia, which occur in the same group of Polyzoa, with corresponding characters in the shells of the typical Brachiopods. He shows that the smaller valve of the Brachiopod is articulated with the larger, just as the operculum of the Polyzoon is united with its cell, or as the lesser valve of an avicularium is articulated with the larger one; while the anus has the same relative direction in both, being in the one case turned away from the operculum of the Polyzoon, and in the other from the smaller valve of the Brachiopod. He further shows that, leaving the pedicle muscles out of consideration, the arrangement of the other muscles is exactly what occurs in an avicularium ; "the adductors, which pass from the larger valve to be inserted into the smaller, in front of its point of support, corresponding precisely with the occlusor muscles of the avicularium ; while the cardinal muscles which arise from the larger valve, and pass to be inserted into the cardinal process of the smaller, behind the point of support, are identical with the divaricator muscles of the avicularium.”
The relations involved in the above views are very interesting. In order, however, to admit them, even to the extent which our ignorance of Brachiopodal embryology will admit, we must cease to look upon the avicularium as a mere organ in the ordinary sense of this word, and consider it as a distinct though very peculiar zooid, homologically repeating the structure of the ordinary zooid, a view against which no valid objections can be urged.
We thus perceive that the affinities of the Polyzoa pass off in two very evident directions -in one through the Tunicata, and in the other through the Brachiopoda ; while the Tunicata and Brachiopoda conduct us by well-marked relations through the Lamellibranchiata into the higher Mollusca.
In determining the limits between the Mollusca and the Molluscoida, Huxley draws the boundary line upon the molluscan side of the Brachiopoda, thus including the latter in the Molluscoida. To this view, however, I have not been able to assent. Notwithstanding the obvious affinities just referred to between the Brachiopoda and the Polyzoa, I yet believe that the Brachiopoda are much more intimately allied to the Lamellibranchiata than they are to the Polyzoa.
The oral arms of the Brachiopoda, in which the most interesting resemblance between this group and the Polyzoa is to be found, while they are undoubtedly related to the lophophore of the latter, have just as decided a relation to the labial tentacles of the Lamellibranchiata, while not only in the condition of the nervous and circulatory systems, which are both upon
* Article “ Mollusca" in ‘English Cyclopædia,' 1855.
a different plan from anything we find in the Tunicata or Polyzoa, but in that of the organi. zation generally, the Brachiopoda possess characters which will scarcely admit of the association of these bivalves with the Molluscoida.*
* I cannot conclude this division of our subject without referring to a most singular little animal, whose true position is undoubtedly among the Annelida, but which in many points repeats the polyzoal form so exactly as to render it at first uncertain whether it be not really a hippocrepian Polyzoon that the observer has under his eyes.
I am indebted to Dr. Wright, of Edinburgh, for an opportunity of examining the little animal in question, of which he obtained two or three specimens from the coast of Devonshire, in May, 1856, and which he preserved alive for some weeks in a vessel of sea-water. Dr. Wright has published an account of it in the 'Edinburgh New Philosophical Journal' for October, 1856, where he describes it under the name of Phoronis hippocrepia. He also kindly placed one of his specimens at my disposal
Phoronis hippocrepia, Wright; the annelidan homomorph of the hippocrepian Polyzoa.
a a. Tentaculiferous crescent. b. Mouth. c. Lip. d. Esophagus. e. Intestine. f. Vessel conveying the blood backwards from the tentacula and crescentic disc. 99. The two vessels from whose union the vessel
h. Vessel conveying the blood forwards towards the crescentic disc and tentacula. ii. Longitudinal muscular fibres in stem.