The Ontogenetic Stages represented by the Gastropod Protoconch.

Transcription

1 GASTROPOD PROTOCONCH. 183 The Ontogenetic Stages represented by the Gastropod Protoconch. By fl. IiCigliton Kestcvcn. CONTENTS. A. INTRODUCTION B. A CONCHOGENETIC NOMENCLATURE.. 1S4 C. APPLICATION OF THE NOMENCLATURE U. A DEDUCTIVE METHOD OF STUDYING THE EAKLY LIFE HISTOEY or A GASTROPOD A. INTRODUCTION. That the so-called " protoconch" may not be the true protoconch has been known ever since Lankester (7) drew attention to the fact that the primitive shell-sac or gland sometimes became filled with a chitinous plug which was subsequently shed, before the inception of the protective shell. The shell which is formed immediately subsequent to this is that which conchologists generally have termed the protoconch; but this also may be shed, and leave behind it an apex, perhaps of several whorls, which differs materially from the rest of the shell, which apex has in turn been.treated as a protoconch. Prom a study of the subject extending over several years, I have come to the conclusion that-a Gastropod protoconch may have been formed during one or more of four stages of growth, and it is even conceivable that it may in some instances consist of portions formed during all four. These four ontogenetic stages are: The typembryonic, phylembryonic, nepionic, a,nd ananeanic, and the portions of the conch formed during each may be advantageously VOL. 49, PART 1. NEW SERIES. 14

2 184 H. LEIGHTON KESTBVEN. termed respectively: Phyloconch, Veloconch, Nepioconch,and Neanoconch. Before proceeding to defiue these conchogenetic stages it may be well to explain why I retain, and use in these definitions, the term "protoconch." The " protoconch," in the accepted and almost universal interpretation of the term, consists of an indefinite number of the apical whorls of a Gastoopod shell, irrespective of whether these apical whoi-ls are the first shell or not. Dall (1) speaks of the Scaphella protoconch, well knowing that the structure so designated was formed subsequent to a horny bulb-like shell, which was discarded in the egg capsule. I have, myself, described the " protoconchs" of numerous species of Lotorium (No. 4), being at the time well aware that what I described were but calcareous casts of the horny veliger shells. Since, then, the term has attained to a general acceptance, to change its significance would immediately cause confusion ; to attempt to discard it altogether would have the same effect. I say " attempt" advisedly, for it is not to be supposed that such a suggestion would receive immediate and universal approbation. Moreover the term is a useful one ; it is here used in the above broad sense, namely, to include those apical whorls, disregarding their age, which by smoothness, distinctiveness, or size are differentiated from the succeeding whorls, and thereby indicate that at their conclusion the mollusc entered upon a new and important ontogenetic phase such, for example, as the change from veliger to sedentary life, or the emergence from the egg capsule. B. A CONCHOGENETIC NOMENCLATURE. Phyloconch. The name is taken from the fact that it is formed by the first distinctive " Phylum " character to appear. It is the "primitive shell" of Lankester, and is formed by almost every member of the Phylum, but, with rare exceptions, is shed at an early stage, and does not enter into the composition of the protoconch. Among Prosobranchs it is retained by Neritina.

3 GASTROPOD PROTOCONCH. 185 Veloconch. The greater portion of this is formed dui'ing the veliger stage, though it may have been begun just before that stage; it is, in fact, that portion of tbe protoconch which is formed subsequently to the flattening out of the primitive shell gland and before the velum commences to be aborted. It may be either corneous, as in Lotorium and Scaphella, and shed at an early stage; or calcareous, as in Triphora, and retained, or lost in the normal and almost universal decollation of apical whorls. Nepioconch. This is formed during the uepionic stage. I have in Number III of ' Notes on Prosobranchiata' given reasons for regarding the varix sometimes thrown up at the conclusion of the veloconch as the only example of a true nepioconch. Neanoconch. The neanoconch, or perhaps more pi*ecisely the ananeanoconoh, is formed during the onfcogenetic stage from which it takes its name. In Lotorium it is moulded inside the horny veloconch. In Scaphella {fide Dall) the veloconch is a round corneous bulb; the neanoconch is not moulded in this, but consists of three or four calcareous whorls formed by the free edge of the mantle, and having a pointed nucleus, which is formed in the veloconch, but not moulded in it. In Megalatractus aruanus (Linn) there are somewhat the same conchogenetic stages j the neanoconch of five whorls is formed by the free edge of the mantle, whilst the corneous veloconch is cast in lime and generally shed before the creature leaves the egg capsule. c. APPLICATION OF THE NOMENCLATUKE. I now proceed to apply this nomenclature to a few protoconchs of which descriptions and figures have been published. In Olausilia, according to Gregenbaur, on the authority of Lankester (7), the primitive shell is retained, so that here the protoconch consists of, at least, phyloconch and veloconch. Lankester, also, mentions other instances of the

4 186 H. LEIGHTON KBSTEVEN. retention of the phyloconch, which it is unnecessary to detail here. In the following instances it is not known whether or not the phyloconch is retained. Murex denudatus (Perry) presents us with a protoconch consisting of veloconch and nepioconch, the latter being represented by a vai'ix (3). The protoconch of Triphoraisa veloconch; a portion of it in some species exactly resembles the adult shell, or true conch (2 and 5). The protoconch of Lotorium is composed as follows : In the veliger stage veloconch; in early neanic stages, neanoconch and veloconch, the former inside the latter; in stages later than ananeanic, of neanoconch only, the veloconch having by that time been shed (4). In Megalatractus aruanus (Linn) the protoconch is a neanoconch, but if it be taken from the egg capsule the calcareous cast of the veloconch will often be found attached to the apical whorl. The shedding of the calcareous cast of the veloconch of this species is particularly interesting; it is parallelled by nothing so exactly as the decay of, and deposit of fresh dentine by, the human tooth. As the outer layer of lime is shed, a fresh layer is deposited inside it. I drew attention to this fact when describing the protoconch, in my recent Report on the anatomy of Megalatractus (6). In Number III of ' Notes on Prosobronchiata' (5), I proposed the term "pseudoprotoconch" for the neanoconch of Lotorium, bub my study of that of Megalatractus has shown me that a protoconch may be a neanic structure and yet not a cast of a corneous veloconch. I therefore wish to withdraw the term, in favour of the more explicit one, " neanoconch." D. A DEDUCTIVE METHOD OF STUDYING THE EARLY LIFE HISTORY OF A G-ASTBOPOD. Except by a study of the early development of the mollusc, it is unlikely that it can be ascertained whether or not the

5 GASTROPOD PROTOCONCH. 187 phyloconch is a component part of any particular protoconch. This method would, of course, be the most satisfactory way of ascertaining the presence or absence of any of the other portions, but there is also a deductive method which may be employed with some degree of certainty. Let me illustrate this method with the example which gave rise to this essay, Megalatractus aruanus (Linn). The large size of the protoconch, as taken from the egg capsule, shows at once and conclusively that the veliger stage was passed within the capsule. What I have here termed the "calcareous cast of the veloconch (and in my description above referred to "veliger shell") is distinctly marked off from the neanoconch by a deep encircling groove. This groove as certainly indicates a pause between two growth-stages as does the varix on the adult shell. The most important pause in early growth is the nepionic stage; it is therefore justifiable to conclude that the groove is the indication of that stage, and that the preceding structure is veliger and the succeeding is neanic. If this deductive method is correct, by its use we shall be enabled to give a brief outline of the early developmental history of any particular testaceous Gastropod. In conclusion, I would draw attention to the fact that the above nomenclature will very likely prove applicable to the Pelecypods equally with the Gastropods. WORKS REFERRED TO. 1. DALL, W. H. 'Trans. Wagner Free Inst. Sci.,' iii, pt. 1, EISCIIEJR, PAUL. ' Manuel de Concliyliologie,' p. 679,1S KESTEVEN, H. LEIGHTON. ' Proc. Linn. Soc. N. S. Wales, xxvi, pt. 4, pp. 709 el seq., KESTEVEN, H. LEICJHTON. Loe. eit., xxvii, pt. 3, pp. 443 et seq,, KESTEVEN, H. LEIGHTON. Loc. cit., xxviii, pt. 2, pp. 443 el seq.., KESTEVEN, H. LEIQHTON. 'Mem. A list. Mus.,' v, pt. 8, LANKESTER, E. HAY. "Mollusca," ' Encyc. Brit.,' edit. 9, xvi, 1885.