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On the Origin, Growth, and Arrangement of Sponge-Spicules: a Study in Symbiosis

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On the Origin, Growth, and Arrangement of Sponge-Spicules: a Study in Symbiosis On the Origin, Growth, and arrangement of Sponge-spicules: A study in Symbiosis. By Arthur Dendy, D.Sc, F.R.S., Late Professor of Zoology in the University of London and Fellow of King's College. With Plates 1, 2, and 3, and 16 Text-figures. CONTENTS. PAGE 1. INTRODUCTORY REMARKS ....... 2 2. SPEGIAL OBSERVATIONS ....... 0 (d) The Stephanotyles of Sceptrospongia coronata . 0 (b) The suppressed Triaenes of Stelletta haeckeli . ft (c) The Formation of Arnphitriaenes by incomplete Twinning 12 {d) The Development of adventitious Rays . .IS (e) The Formation of nodal Whorls in Discorhabds . 18 (/) A revised Account of the Development of the Anisodisco- rhabd in Latrunculia bocagei .... 27 3. GENERAL CONCLUSIONS AS TO THE ORIGIN AND GROWTH OF SILICEOUS SPONGE-SPICULES . .... 36 (a) The primary co-operating Agents . 3(> (6) The Influence of the primary Protorhabd upon the Form of the Spicule 3!> (c) The Influence of the Mode of Division of the original Scleroplastid . .41 (a) The Formation of radiate Spicules . .41 (|S) Variation in the number of primary Rays . 43 (y) The Formation of Dragmata and Rosettes . 44 (d) The Influence of adventitious Growing Points upon the Form of the Spicule 40 (a) Adventitious Growing Points associated with Protorhabds 40 (|3) Adventitious Growing Points without Protorhabd Formation ....... 47 NO. 277 B 2 ARTHUR DENDY PAGE (e) Mechanical Influences affecting the Form of the Spicule . 49 (n) The Conversion of dermal Spicules into Plates . 49 (j9) The Mechanical Influence of the so-called Mother- cell 50 (y) Other Mechanical Influences . 51 4. COMPARISON OF THE DEVELOPMENT OF CALCAREOUS SPONGE- SPICULES 52 5. COMPARISON or THE SPONGIN SPICULES OF DARWINELLA . 53 6. CONSIDERATIONS AS TO THE NATURE OF THE SCLEROPLASTIDS . 54 (a) Their microscopical Characters, Mode of Growth, and Multi- plication ........ 54 (6) Their Specificity ........ 5.r» (c) The Occurrence of trimorphic Chelae and its Significance . 50 (d) The Want of Correlation between the Spicules and the Sponge 57 (e) The Dropping out of Spicule Categories in Sponge Phylo- geny ......... 58 (/) The sporadic Distribution of certain Spicule Types . 60 (g) The Scleroplastids as symbiotic Organisms (Sclerocoeci) . 01 7. THE RELATIONS BETWEEN THE SOLEROCOCCI AND THE SPONGE . 02 8. THE TAXONOMIC VALUE OF SFONGE-SPICULES . .07 9. LIST OF LITERATURE REFERRED TO 09 10. DESCRIPTION OF PLATES ....... 72 1. INTEODUCTOHY KEMAHKS. THERE are perhaps no structures met with in the animal kingdom of which the origin and significance are more diffi- cult to explain than the spicules that form the skeleton of calcareous and siliceous sponges. The larger and usually less complex forms, technically known as megascleres, we are apt to accept as necessary constituents of the skeleton specially adapted to their position in the sponge, without stopping to reflect that there are many sponges which manage to get on very well without any spicules at all, sometimes, indeed, with- out skeleton of any kind. The smaller ones, or microscleres, met with amongst the siliceous sponges, are much more enig- matical, for they usually occur scattered promiscuously through- out the soft tissues, while they exhibit the most surprising variety SPONGE-SPICULES 3 of exquisitely symmetrical forms, the meaning of which it is impossible to interpret in terms of the physiological require- ments of the sponge. At the same time these forms, as well as those of the megascleres, while exhibiting a remarkable degree of specific constancy, can be arranged in beautifully graduated series that leave little doubt in the mind of the trained observer that they owe their existence to some evolutionary process. The evolution of many of these series has been discussed in some detail in my memoir (1921 a) on ' The Tetraxonid Sponge- spicule : a Study in Evolution ', to which I would refer the reader who is unacquainted with the subject for descriptions and illustrations of many of the more remarkable types. I must, however, point out that the illustrations in that paper do very scant justice to the extraordinary beauty of the forms described. Much more artistic and at the same time more realistic representations of sponge-spicules are to be found in numerous monographs that deal systematically with this group of the animal kingdom, the best being perhaps those given by Schulze in his various memoirs on the Hexactinellida (1887 c, 1894 6, 1895, 1899 a, 1904, &c.) and by Lundbeck (1902, 1905, 1910) in his reports on the Porifera of the Danish Ingolf Expedition. I would refer the reader to the beautiful plates accompanying these memoirs, and to those illustrating my own reports on the sponges of the ' Sealark ' Expedition (1916 b, c ; 1921 b) and of the ' Terra Nova ' collection (1924 a) for some idea of the nature of the problem before us. Various attempts have been made by different investigators to trace the individual development of certain types of sponge- spicule. Perhaps the best known of these are Minchin's classical observations on the development of calcareous spicules (1898, 1908), to which I shall have occasion to refer later on. In the meantime I may venture to say that these observations seem to me to stop just at the most critical point. As regards siliceous spicules, the few observers who have paid much attention to the subject have usually been content to say that they originate within ' mother-cells ', and to give some details as to their growth stages. The so-called mother-cell has served B 2 4 ARTHUR DENDY as a convenient shelf on which to lay aside the problem of the origin of the spicule (compare Woodland, 1908). In 1917, in my Presidential Address to the Quekett Micro- scopical Club, I put forward certain views as to the origin and growth of siliceous spicules which formed the starting-point from which those enunciated in the present memoir have gradually developed. I pointed out ' that three factors may be concerned in the growth of a siliceous sponge-spicule : (1) the protorhabd, which is responsible for growth in length and serves as a foundation upon which silica is deposited ; (2) the formative cells, which are responsible for the arrange- ment of the silica ; and (3) the accessory silicoblasts, which supply the formative cells with the necessary material.' The name ' protorhabd ' was coined by me for the ' axial thread ', which had long been known to occupy the ' axial canal' of the spicule, regarded from the developmental point of view. The so-called formative cells were very problematical entities, which had been observed only in the case of L atrun- culia bocagei. In my memoir on the tetraxonid sponge- spicule (1921 a) I pointed out that although they might pro- visionally be spoken of as ' initial cells ' their real nature was somewhat doubtful, the condition of the material precluding exact cytological observations. According to the views to be elaborated in the following pages they are not to be regarded as themselves cells, or even as representing true cells, but as minute growing points, associated with the presence of still more minute granules, which seem to form the starting-points in spicule development. If we regard these granules merely from the point of view of their functioji as spicule initiators, we may call them ' scleroplastids ', on the analogy of chloro- plastids and other similar bodies in the vegetable kingdom, but if, as seems to me probable, they are really symbiotic organisms, then, from this point of vieAv, the name ' sclero- cocci' would seem to be preferable. Whatever we call them, they must be recognized as the foundations (usually elongated into protorhabds) around which silica is deposited by fchd silicoblasts of the sponge. These silicoblasts, temporarily or SPONGE-SPICULES 0 permanently enveloping the growing spicule, are the ' mpther- oells ' of earlier observers. The scleroplastids or sclerococci are so extremely minute, being of the same order of magnitude as bacteria, that it is difficult, if not impossible, to recognize them before a certain amount of silica has been deposited around them, for aa yet I know of no means by which they could be distinguished from TEXT-FIG. 1. r--- Young anisodiscorhabd of Latrunculia apicalis with projecting protorhabd (pr.). x 1,187. ordinary bacteria and other granules that occur abundantly in the mesogloea. When they have elongated to form pro- torhabds, however, they are more readily detected, though even then it is doubtful if they have ever been seen absolutely naked, without any silica deposit. Apparently they are repeatedly enveloped (more or less completely) by the phago- cytic silicoblasts, which usually, after depositing their silica, again leave them lying free in the mesogloea. The actual existence of the protorhabd as an independently growing entity is clearly demonstrated in Text-fig. 1, which represents a young anisodiscorhabd of Latruncnlia a pi- 6 ARTHUK DENDY calis, with the protorhabd projecting freely from the axial canal at the apex of the spicule. The protorhabd stains freely with such reagents as borax carmine and brazilin, and may then often be detected as a very slender, darkly stained thread, lying in the axial canal of the young spicule. The evidence that it is derived from a scleroplastid, and that the latter is a quasi- independent organism, capable of moving from place to place, multiplying by fission, and perhaps sometimes undergoing sexual union, will be set forth in the following pages. Much of this evidence is now published for the first time, but, in order to make the story as complete as possible, I have ventured to recapitulate certain facts, and to reproduce certain illustra- tions, already published in the papers referred to above. 2. SPECIAL OBSERVATIONS. (n) The Stephanotyles of Sceptrospongia coronata. In the genus Raphidotheca, amongst the Esperellinae, a very curious modification of the ordinary tylostylote mega- sclere, such as we find in Mycale and Esperella, gives rise to the exotyle.
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