On the Morphological Relationships of the Molluseoida and Ccelenterata, and of Their Leading Members, Inter Se

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XXXVIII.—On the Morphological Relationships of the Molluseoida and Ccelenterata, and of their leading Members, inter se. By JOHN DENIS MACDONALD, R.N., F.R.S., Surgeon of H.M S. "Icarus."

(Read 21st December 1864.) Few departments of zoology have recently suffered more remarkable changes, both in classification and accepted views of structure, than the Polypi or Ccelen- terata, and their immediate allies in the ascending scale, the Molluseoida,—greatly depending upon the more extended study of those animals of late years. We have been thus enabled to discover natural affinities which prima facie evidence would scarcely ever have indicated, as well as intrinsic differences which the same kind of evidence has hitherto been incapable of revealing to the mind. Leading from the Protozoa to the Mollusca proper, the Coelenterata and Mol- hiscoida constitute an unbroken series of animals forming a considerable section of invertebrata, distinguished from the Protozoa by the development of true ova, and from the Mollusca by the property of gemmation developing compound ex- amples of the principal types. Furthermore, the motion of the blood, or its equivalent, is effected either by ciliary action or by a propulsive organ; but when the latter occurs it is unfurnished with valves, so that the course of the circulation may be reversible in the same canals. The study of the different stages of development of a certain organ in the same animal comes within the pale of ordinary physiology: but when we pry into the progressive advance of any organ or function, taken in the abstract, we enter upon a more comprehensive branch of science, which not only embraces the common physiology of each particular animal, but its combined import in all. On comparing the relative parts of two distinct animals, one unaccustomed to a study of this kind could scarcely doubt that the mouth of one was anything more or less than the exact equivalent of the mouth of the other; but it may be clearly shown that mouths acting as such, so far as simple function is concerned, may nevertheless exhibit a remarkable difference, homologicallv speaking, in animals constructed on different types. It was formerly believed that the branchial and cloacal orifices of the Asci- dian were homologous with those of the siphonal tubes of Lamellibranchiata; but this very natural error has been pointed out by Professor HUXLEY, and it can- not be doubted that the orifice of ingress is in reality oral in one, while it is simply pallial in the other. In the Ascidian, moreover, the inner or commonly recog- nised mouth is but the oesophageal opening, though it is critically answerable to VOL. XXIII. PAET II, 7A 516 MR J. D. MACDONALD ON THE MORPHOLOGICAL RELATIONSHIPS the mouth of the polyzoon. It may be assumed that the prominent mouth of the Hydrozoon (in which, in truth, there is no stomach homologous with that of the higher types) is equivalent to the everted internal gastric opening of the Acti- nozoon; or, conversely, that the stomach of the Actinia is but an inversion of the oral projection of Hydra, still preserving a communication with the somatic cavity, but necessitating the formation of a new oral orifice. In the same way, the tentacula encircling the mouth, in some of the lower forms of animal life, are not in all instances homologous organs. Thus, the branchial tentacula, as they occur in the Ascidiozoa, are found in none of the other members of the series now under consideration; and in the passage from the tubularian Polyp to the Actinozoon, the oral tentacula of the former (with a single exception, so far as known to me) are suppressed in the latter; while the outer or somatic set remains, and even becomes more numerous or densely crowded as a general rule. The true nature of the Aggregate Tunicata was first made known by SAVIGNY, and they were with great propriety removed from the zoophytes, with which they had been formerly confounded. To M. MILNE-EDWARDS is due the credit of having elevated the Polyzoa from their low estate, and ranked them with the Tunieata in his Molluscoid group. Professor HUXLEY again, from his comprehensive view of the subject, saw the propriety of associating the Brachiopoda with the Molluscoida, but more immediately with the Polyzoa, as exhibiting the "neural" intestinal flexure, in connection with many very striking points of homology which had never before been conceived.* The delicate membrane surrounding the base of the tentacula in the Polyzoa Hippocrepia is considered by Professor ALLMAN as analogous to the membrane of the respiratory sac in Tunicata ; but Mr BUSK says that this has not yet been detected in any marine Polyzoon, though I may add that it is distinctly present in the Brachiopoda; and he further considers that the membrane surrounding the base of the tentacula in Pedicellina is not homologous with it, having an entirely different import. If ever a Polyzoon resembled a Tunicary, it is the said Pedi- cellina, more especially when its two dorsal bends are in course of development, and a zealous observer might be very readily deceived as to the true nature of certain parts in one bearing a striking but delusive resemblance to those in the other. Yet, without going farther into the refinement of the subject, it would not be far wrong to assume, in round terms, that the pharyngeal respiratory system of the Tunicata is represented by the oral tentacula of the Polyzoa.^ The epistome of the Polyzoon, moreover, is regarded by Professor ALLMAN as homologous with the languet of the Tunicata.

* It will be seen, in the course of this paper, that I have availed myself of the useful terms employed by Professor HUXLEY, more particularly with reference to the Ctelenterata. •f See Professor ALLMAN'S remarks on this subject, in his valuable work on the fresh-water Potyzoa. Published by the Ray Society. OF THE MOLLUSGOIDA AND CCELENTERATA. 517

The property of gemmation in a marked manner distinguishes the Mollus- coida from the Mottusca proper, on the one hand, while it associates them quite as obviously with the Coelenterata on the other. We know nothing of a process of this kind as occurring in the so-called Ctenophora, including the families Cal- lianiridce* and Beroidce; but the organisation of these animals, first rightly considered by FEEY and LEUKART abroad, and by Professor HUXLEY at home, referred them to the higher section of Coelenterata, namely, the Actinozoa. Yet, the more I have studied Cydippe, the more it has appeared to me to hold a position between the Actinozoa and the Polyzoa, linking the two by characters which it exhibits in common with either, or both. Indeed, in any other place it would seem to be not only friendless but intrusive. A view similar to this has, I believe, been already expressed by M. VOGT, but I regret that I have not access to his observations on the subject. It is curious to observe the progressive development of the digestive system, proceeding from the Hydrozoon onwards through the Actinozoa, Ctenophora, and Polyzoa, to the Tunicata. The important part taken by the Ctenophora as a link in this beautiful chain, is represented in the accompanying series of diagrams. Moreover, certain points in the structure of these animals, are even made more tangible to our philosophy by the light which is thus shed upon them. The stomach of Cydippe is connected with the walls of the body by two vertical septa, with two interseptal loculi. The ccecal tubes forming the lining of these loculi run forwards as far as the mouth, and are at once diverticula of an alimentary system and of a somatic cavity. We observe here, as in Actinia, a well defined internal gastric opening, bounded by two crescentic folds, determined by the persistence of the before-mentioned loculi. These latter communicate below with the rudimentary intestine which is yet little more than a central tubular narrowing of the somatic cavity, from the gastric end of which also pass off the two dichotomously-branched tubes, which terminate peripherily in the fusiform sinuses.f corresponding with the ciliated bands. All this affords us a more distinct idea of the mode in which the intestinal tube is formed. Thus, instead of arising simply as an extension of the proximal end of the stomach, the whole gut is at first but a tubular process of endoderm, inclosing a portion of the somatic cavity. In Cydippe, the intestine is perfectly straight and axial, reaching the posterior extremity of the globose body, where it exhibits a small, but distinctly marked bifurcation, and the little nervous ganglion, with its otoconical sac, is received into the intervening recess.

* This family name is objectionable, as having been chosen from.a supposititious genus founded upon a mutilated specimen of Cydippe, indifferently drawn. f Would it be too far-fetched to suppose that these sinuses are, as it were, retrospective of the tentacula of Actinia ? 518 MR J. D. MACDONALD ON THE MORPHOLOGICAL RELATIONSHIPS

The end of each division is well rounded, and closely applied to the ectoderm on either side of the ganglion, but I have always found it difficult to detect the so- called anal openings, though I have once or twice observed the escape of matters from within the tube at this part. The least I can say, however, is, that they are by no means so definite in nature as they are represented to be in figures and descriptions. Professor HUXLEY first pointed out the striking original difference in the intestinal flexure in Brachiopoda and Polyzoa, as compared with that in Tunicata. Thus, in the two former, the intestine is simply flexed forward {i.e., towards the nervous ganglion or "neurally"), while in the latter it is at first flexed backwards (i.e. towards the heart, or hsemally), and subsequently turns forward to terminate neurally in the " atrium.'1'' In Cydippe, therefore, the intestine is straight; in the Polyzoon and Brachiopod, it is once flexed (forwards), and in the Tunicary it is twice flexed (backwards and forwards). The nervous ganglion appears to hold some relation to the conditions just noticed ; thus, it is nearly midway between the branchial and cloacal openings in the Tunicary, between the oral and anal orifices in the Polyzoon, and at the posterior termination of the intestine in Cydippe. The homologies of the ciliated bands and tentacula of Cydippe have not yet been satisfactorily determined, though it does not appear improbable that the former organs represent the tentacula of the Polyzoon or of the Brachiopod having become retroverted, and connate as it were with the body, as next in order of suppression. The configuration of the genus Cestum is strongly in favour of this view, and if the possibility of such be admitted, then the retractile racemose tentacula may be regarded as pallial. If, on the other hand, the position here supposed in relation to the ciliated bands of the Ctenophora be doubted without valid reason, I can only say that it is not at all more remarkable than the modifications of the ambulacra in the Echinodermata, passing from Asterias to Spatangm for example. The branchial tentacula in the Tunicata may be regarded as ecclusory and perhaps prehensile, while respiration is effected by their pharyngeal apparatus, and the same office is probably also exercised bjr its homologue the oral circle of tentacula in the Polyzoon, and the so-called cirri of the Brachiopod. In the case of the latter, however, it may be subserved also by the pallial sinus system, which is ciliated within, and thus enabled to circulate the contained corpusculated fluid. The renewal of the water passing over it is of course chiefly brought about by the action of the vibratile cilia clothing the double row of tentacula (" cirri"). There is apparently, to my mind, but a short step from these latter organs to the ciliated bands of Cydippe, in which they are both locomotive and respiratory, being in close relation with the aquiferous system; and it must be remembered, that this, like the pallial sinus system of the Brachiopod, is not only lined with OF THE MOLLUSCOIDA AND CCELENTERATA. 519 cilia circulating a corpusculated fluid, but also contains the reproductive organs. Finally, the tentacula in the Coelenterata are in most cases both prehensile and respiratory, while in many they are also locomotive. The accompanying series of diagrams will show more distinctly the relation- ship of the various organs and parts in the five groups so imperfectly glanced at above. A. c. :• ,-4

3—

4--

D. E. F. G.

A. An Ascidian. E. Tubularia. B. A Polyzoon. F. Clava. C. Cydippe. G. Hydra. D. Actinias, fixed and free.

The numerals have a corresponding signification in all the figures. 1. The outer oral, or branchial tentacula of Tunicata, which do not appear again in any of the inferior types. 2. The outer mouth or branchial opening of Tunicata only to be represented theoretically, by the conjoined summits of the tentacula of Polyzoa. 3. The branchial chamber or respiratory pharynx of Tunicata, to which the tentacula of Polyzoa and the ciliated bands of Cydippe may offer an equivalent; being thus pharyngeal in the first, oral in the second, and somatic in the last. It is, moreover, altogether absent in the other sections of Codenterata. 4. The inner or cesophageal opening of Tunicata, and the proper mouth of Polyzoa, Ctenophora, and Actinozoa. 5. Pyloric orifice of the stomach, leading into a closed intestine in Tuniaata and Polyzoa; and into an intestine communicating with the somatic cavity in Ctenophora ; and solely into the somatic cavity in the Actinozoa. Probably, also, in the everted state forming the oral orifice in Hydrozoa. 6. Anal extremity of the intestine in Molluscoida and Ctenophora. 7. Position of the nervous ganglion in the same. 8. The oral tentacula in Actinozoa, probably equivalent to the outer or somatic tentacula of the Hydrozoa, always occurring on the outer or proximal side of the reproductive organs. 9. The oral tentacula of Hydrozoa, always on the distal side of the young gemmules. In accordance with the views here expressed, it may be assumed that the inner circle of Tubularia is homologous with the single series of Hydra, &c.

Regarding Cydippe as holding a middle position in the foregoing series, the VOL. XXIII. PART II. 7 B 520 MR J. D. MACDONALD ON THE MORPHOLOGICAL RELATIONSHIPS theoretical passage of the type upon which it is constructed into higher and lower types is effected in the tentacular system by addition, and in the alimentary system by modification, and these are, of course, either progressive or retrogressive, as they tend to the higher or lower forms. Thus, the tentacula of the Poly soon, and the branchial tentacula of the Ascidian, are progressive additions, while the oral tentacula (so-called) of the Actinosoa, and then the oral tentacula of the Hydrozoa, are retrogressive additions, however contradictory the phrase may appear.* On the other hand, the insulation and simple flexure of the intestine in Polyzoa, and the perfect looping or double flexure of the intestine in Tunicata, are progressive modifications, while the simplification and central en- largement of the somatic cavity, abolishing the intestine in Actinozoa, and, finally, the virtual eversion of the stomach in Hydrozoa, are retrogressive modifications. To make the principles here expressed more intelligible, I have arranged them in a tabular form, thus,— Conversions of the Cydippean Type. I. MODIFICATIONS AFFECTING THE ALIMENTARY SYSTEM.

* Progressive. Complete insulation of the intestine from the somatic cavity. a. Primary type, with simple neural flexure—Brachiopoda and Polyzoa. b. Secondary type, with primary hsemal and final neural flexure—Ascidiozoa. ** Retrogressive. a. Primary type. Stomach more completely insulated by the abolition of the intestine— Actinogoa. b. Secondary type. Eversion and consequent abolition of the true stomach—Hydrozoa.

II. ADDITIONS AFFECTING THE TENTACULAR SYSTEM. * Progressive. a. Primary type. Oral tentacula of Brachiopoda and Polyzoa. b. Secondary type. Branchial tentacula of Ascidiozoa. ** Retrogressive. a. Primary type. Oral tentacula* of Actinozoa. b. Secondary type. Oral tentacula of Hydrozoa. Two suggestions present themselves in this mode of considering the subject,— viz., first, that the Brachiopoda and Polyzoa should be taken together as a group in themselves, quite as natural as the Tunicata, consisting of simple and compound forms; and, secondly, that the Ctenophora should be received within the pale of the Molluscoida, perhaps as a pelagic section representing Salpa, Doliolum,

* It will be observed that the homologues of the oral tentacula of Tubularia do not exist in Luoernaria, while they are the sole prehensile organs of Goryne and Hydra. OF THE MOLLUSCOIDA AND (XELENTERATA. 521

&c, amongst the Tunicata. In accordance with these views the following classification may be given:— ( With primary haemal) . .,. Intestine insulated from J and final neural flexure, j Ascldl0z0a- Molluscoida (including the somatic cavity, . ^ With simple neural") Brachiopoda the Ctenophora), ' flexure, j and Polyzoa. Intestine straight, and communicating with the ) somatic cavityi, } ( Intestine obliterated; stomach communicating with \ . . Ccelenterata (excluding J the somatic cavity j the Ctenophora), . . ] True stomach obliterated, its office being answered 1 TT , ( by the somatic cavity, \m™zo*. * These are probably in ordinary cases truly somatic, for, in some instances, an inner and very distinct set exists, more worthy of the name of oral, homologically speaking.