1846.] REPRODUCTION OF MEDUSE. 25 rations, whose function it is, as it were, to prepare the way for the later, succeeding generation of animals destined to attain a higher degree of perfection, and which are developed into the form of the mother, and propagate the species by means of ova, can, I believe, be demonstrated in not a few instances in the animal kingdom." P. 2. His investigations concern more especially the phenomena which have been observed in the propagation and development of the Medusa and of the Claviform Polypes among the Acrita, of the Salpæ among the Mollusca, and of the Trematoda among the Entozoa. The mode of generation in the Medusa has for a long time been an enigma among physiologists, and although much of this obscurity was cleared up by the admirable researches of the Norwegian naturalist Sars, and subsequently by those of Siebold, yet it was reserved for the author to trace all the links which connect the ovum with the fully-formed animal. The eggs, after experiencing a certain degree of development first of all in the ovaries, and subsequently in the marsupial vesicles belonging to the oral arms or tentacles, disclose, and bring forth young, not, as might be expected, having the characters of the parent Medusa, but creatures of an oval form, and outwardly resembling the Ciliated Infusoria, such as Leucophrys or Bursaria. This apparent, but not real, infusorial animalcule soon attaches itself to some fixed object, and by degrees becomes transformed into an animal, having a mouth provided with a number of tentacula amounting ultimately to thirty; in this stage it very much resembles a polype. By a kind of transverse fission this polypiform animal is gradu ally separated into many portions, which, acquiring independent vitality, are finally separated from each other, and become free swimming animals. Now it is within the abdomen (if such a term may be here applied) of these polypiform Medusæ that the true larvæ of the perfect animals are found; and we must, as far as we at present know, assume that the function of these polypiform individuals is fulfilled when the larvæ have reached a definite development, and that their whole existence has for its object the perfecting of a series of beings of the same species, to which they stand in the relation in what, for want of a better term, may be called "foster-parents." (P. 24.) M. Steenstrup concludes his observations on the propagation of the Medusæ by stating, that— "Since, according to the observations of both Sars and Siebold, only animals which have the power of affixing themselves proceed from the ova of Medusa, all of which become in the way so often described, polypiform nurses,' which nourish the Medusa-larvæ from their bodies, a considerable anatomical and physiological difference clearly exists between them, which are all of one sex, and the perfect Medusa which are of two sexes. Here, however, we observe a great natural harmony; since wherever we find the fostering of a brood or fry to be committed to special individuals, these are always of one sex, and indeed females in whom the sexual organs of germination remain in an undeveloped state, whilst in consequence of this abortion in the development of these parts, the instinct or nisus serving for the preservation of the species takes a peculiar direction.” P. 24. This mode of development has only been seen in two of the most common species of Medusa (M. aurita and Cyanæa capillata,) but, in all probability, it occurs throughout the whole family. It is further proper to remark that, although the author admits with Sars that the "nursing" individuals increase by gemmation, every part of the body being capable of throwing out gemmæ or buds, yet he contends that this fact does not affect the truth of his representation; for the question arises-May not an ovum occasionally contain several embryos, one of which becomes developed at the expense of the others, which make good their individuality at a later period?-(L. c.) We are not, it is true, in a position at present to solve this question; but it must be apparent to those who are acquainted with the obscurity overhanging the reproductive process among the most simple animals, some of which are believed to establish the doctrine of equivocal generation, whilst others are pronounced at one time to be fissiparous, at another gemmiparous, and at another oviparous, that these researches of Steenstrup, if ultimately established, must render a review of the whole theory of generation indispensably necessary. The next class investigated, referring to some families of the multitudinous section of polypes, throws further light upon this interesting question. We shall select for illustration one of the bell-shaped polypes, Campanularia geniculata. On the branched stem of one of these individuals, are placed a number of cells or cups, shaped like inverted cones or bells, which have a determinate position, and are of three kinds, constituting, according to the author, so many distinct generations. Those of the first order occupy the summits of the branches, and elicit afterwards the larger individuals forming the second class, which, like the bud in a plant, are situated in the axillæ of the branches; in the interior of these axillory polypes are placed the individuals of the third kind or generation, which are in fact the perfect, that is, the reproductive females. The mode in which this curious progeny is produced is, according to Steenstrup, as follows:-between the outer and inner membranes of the second order (axillary polypes) and upon the so-called intestinal tube, small vesicular elevations are formed, each of which becomes included in a transparent membrane or sacculus; within this sac, and in connection with the vesicular elevation, two perfectly spherical bodies appear; subsequently, the sacculus with its contents approaches the operculum of the axillary cell or polype, and breaks through it, without, however, losing its connection with the central tube; finally, the apparent sacculus opens, a circlet of about twelve irregularly-toothed tentacula projects around the opening, and it is now seen that the sacculus is in reality a distinct individual. Several such individuals are found within a single axillary polype; and we may now state that they are the completely developed females above noticed, the two spherical bodies being prolific ova, presenting opaque granules (yelk-globules?) and, which is more interesting, the germinal vesicle of Purkinjé. The subsequent steps of the process require to be carefully attended to:-the ova are developed within the parent-body, and when the embryo is matured, it ruptures the delicate membrane of the egg, remains for some time within the parent animal, and then passing out, it enters the water, freely swims about for a time, being provided with vibratile cilia, afterwards attaches itself to some fixed object, and becomes the origin of a new compound polype, on which are formed the three cup-like cells already described. Thus, according to the views here advocated, three distinct and successive generations are en 1846.] REPRODUCTION OF SALPÆ. 27 gaged in perfecting the fully-developed and fertile female, and are so many sets of nurses or foster-parents. "Upon reviewing this sketch, it can hardly be considered to present an instance of metamorphosis, since it is not the same individuals which in the course of time exhibit various forms, but we see here also, a series of generations whose succession in a definite order is necessary to the complete development of the species. The ciliated embryos which, by their adhesion lay the foundation of the polypes, originate only from ova, which are developed in the females. These latter are generated between the inner and outer tunic of the polype in the axillary cells, which polypes may consequently be considered as the nurses' or foster-parents of the female individuals, whilst they are themselves, on the contrary, derived from a previous generation of polypes of a different form, which occupy the extremities of the branches, are of the same shape as the polype of the stem, and must be regarded as of the same kind, although they appear to arise from it by what is called gemmation.' P. 37. In the preceding instances, the theory of Alternate Generations has been traced only among the Radiate sub-kingdom, and consequently in reference to the simplest animal forms. In the Third Chapter, however, comprising an account of the Alternate Generation of the Salpæ, the author proceeds to a more important division, that of the Mollusca. It may be as well to premise that the Salpæ belong to that order of the acephalous molluscous animals, called Acephala nuda, and which are divided by Cuvier into two families the solitary and the aggregated-according as the individuals are met with in an isolated or aggregate condition. We may further remark that there are some peculiarities connected with the mode of generation among these naked Acephala, which have for a long time occupied the attention of Naturalists, and have given rise to many conflicting opinions. We will, in the first place, state the facts that have been ascertained, and then give M. Steenstrup's interpretation of them. It is well known, as regards the Salpæ, that although they are classed by Cuvier in the solitary division, yet they are also met with joined together, forming long chains consisting of from 20 to 40 individuals or more, possessing organs for attachment; these salpa-chains are found in the Mediterranean and warmer parts of the ocean, and all the individuals forming them move with great uniformity, so that the whole chain progresses in a serpentine manner below the smooth and tranquil surface of the water. The most interesting facts hitherto known respecting these curious mollusks were observed by Chamisso. That excellent naturalist ascertained that the solitary salpæ, though never themselves forming parts of a chain, yet always contain a progeny resembling those which do, and, further, that each separate embryo of this progeny is linked together like the individuals forming the salpa-chains; whilst in the associated Salpæ, on the contrary, he found young resembling the solitary ones. "Now, as examination of the free swimming individuals, which presented marks of having been disjointed from a chain, proved that these also contained only solitary pedunculated embryos, Chamisso concluded from his observations, that all solitary Salpa produced associated ones or chains; and, on the contrary, that all the associated Salpa were parents of solitary ones, and these again of the associated, and so on." (P. 39.) This theory of the alternately solitary and associated generations of the Salpæ was so opposed to all then known of the reproductive process, that it is not a matter of surprise that it encountered much opposition. Professor Eschricht, of Copenhagen, although one of Chamisso's opponents, admitted his facts, and added one strongly confirmatory of the truth of his deductions, namely, that in the case of the Salpa Cordiformis, “each embryo constituting the embryo-chain, while still enclosed in the body of the mother, presented distinct traces of younger solitary embryos." It is not our object to enter into any of the details of this controversy; we will therefore only mention, as showing the fallacy of Professor Eschricht's arguments, that he supposed the Salpæ, whilst in the associated form, and therefore certainly in an immature condition, might give birth to solitary individuals, and subsequently, becoming detached from each other, that they produced a later generation of associated embryos; a supposition which, as Steenstrup justly remarks, is opposed to the fundamental physiological principle, that no animal is able to propogate before it has attained its perfect form and development. Such, then, being the fact, for we assume as proved the position of Chamisso, that all Salpa-chains are produced from solitary Salpæ, and all solitary Salpæ from the associated individuals, we come to the author's interpretation. After pointing to the resemblance between the phenomena just described and those noticed in the propagation of the Medusa, and which assuredly cannot escape observation, M. Steenstrup proceeds "I have consequently no hesitation in expressing my belief, that the alternate generation of the Salpa from solitary and associated individuals, is to be explained in the same way as that of the Medusa, and that the first generation of these animals serves as a series of foster-parents, whose object is, by their furthering the development of the second generation, to conduce to the perfection of the species. I am, however, prepared to admit, that this kind of explanation does not enlarge our knowledge with regard to the propogation of the Salpa; I only think, that the phenomena already known, will have a clearer light thrown upon them from the analogy. Some doubt even must still remain, on the circumstance whether the solitary or the associated Salpa are the foster-parents. In my opinion, however, it is the solitary form rather than the other which should be assumed to be that of the nurses.'" P. 46. In this last conclusion we cannot agree with the sagacious author, inas. much as, according to his own remark, the free moving animal must be regarded as more perfect and developed than those which are attached together, and, consequently, as being the true prolific individuals or parents. 66 We have now to notice one of the most interesting parts of the whole inquiry, the development of the Trematode Entozoa, and especially of the fluke or Liverworm (Distoma Hepaticum.) Although some links are still wanting, sufficient proofs are adduced to show that the system of nurses obtains here, as in so many other of the lower classes of animals. By a very minute and comprehensive investigation, the author appears to have ascertained that several of the Trematoda when young are not connected with any organ, but enjoy free locomotion in water, externally to the animal, which in their future state they infest-(he particularly has examined the flukes of water-snails, such as Limnæus stagnalis, Planorbis cornea,&c.) "In this condition they are provided with a locomotive organ, usually a tail of moderate length, by the waving movement of which the creature propels itself 1846.] REPRODUCTION OF ENTOZOA. through the water, like a tadpole, to which in its external form it is not dissimi except that it is much smaller, and almost microscopic. In this larval state, Trematoda are known to naturalists under the generic name of Cercaria. It well known that this form was not a permanent one, but we were ignorant of changes which it underwent. As I have been so fortunate as to trace th changes, I will now detail them." P. 53. This, then, is the first connecting link in the investigation, namely, t Cercaria are in reality the true larvæ of the Distomata infesting wat snails; but they undergo a curious metamorphosis before attaining the ultimate and mature character. On examining with a sufficient magnif ing power a portion of the skin of the snail, to which the Cercariæ adhe by their abdominal sucker, it is seen that they make forcible lashing m tions of the tail, which organ at length is thus cast off as a lifeless mas when "the tailless animal itself assumes so completely the appearance a Distoma or fluke, that it could not fail of being recognised as belongin to that genus, in case it were met with in this condition in the viscera other animals." Subsequently the animal, burying itself in the soft int gument of the snail, becomes covered with mucus, (which graduall hardens and becomes a nearly transparent case); it is now in fact a pupa and remains in that state for a period of from two to nine months, when quitting its puparium, it becomes a true entozoon, the Distoma Hepaticum We have here the solution of the enigma which puzzled the celebrate helminthologist, Siebold, who could not ascertain what became of the pupa of the Cercariæ. These points having been determined, it is necessary next to state tha the Cercariæ, although they are thus proved to become Distomata, are no themselves immediately derived from those trematode animals, but from certain parasitic worms found in snails, and described by Bojanus under th somewhat barbarous name of "King's-yellow-worms," and which are there fore, in the language of Steenstrup, "nurses," (Altrices.) This derivation would seem sufficiently complex, but the author is satisfied that even thes nurses do not proceed from the ova of the fluke, but from an interpose and pre-existing brood (termed parent-nurses, Abaltrices,) in the interio of whose bodies he plainly distinguished "a progeny consisting of actua nurses in all stages of development."(P. 69.) The precise origin o these parent-nurses has not been determined, although it is said there i no doubt they are the immediate offspring of the parent Distoma, bein derived in the ordinary mode from ova. In the following table we hav endeavoured to place this strange genealogy in a clearer point of view:1. Distoma Hepaticum-perfect animals, androgynous, reproductive. 2. Parent-nurses (Abaltrices) derived from ova Imperfect 3. Nurses (Altrices,) King's-yellow-worms 4. Cercariæ, true larvæ of fluke. females. The author does not describe with any minuteness one of the most im portant points in the whole theory, the true nature, namely, of the gern from which each generation springs. He says, " at first the germs, fron which the Cercaria are developed, are nearly spherical, and appear to b formed of numerous vesicular globules or cells, which are pretty clearly seen to be all surrounded by a very delicate common membrane: thes spherical germs afterwards increase in length, as they are formed int |