Pedal Disc Research Articles

Inhibition and Rhythmic Activity of the Circular Muscles of Calliactis Parasitica (Couch)

ABSTRACT The responses to electrical stimulation of isolated rings of the column and pedal disk of Calliactis are described. Such rings make slow spontaneous contractions which are frequently rhythmical, the interval between contractions normally being 7-20 min. Continuous low-frequency stimulation inhibits spontaneous activity of rings from the pedal disk and also of fresh rings from more adoral regions of the column. Older rings from the mid-column respond to such stimulation by a tetanic contraction. The latency of response to electrical stimulation of pedal rings is of the order of 120 sec. This latency is not affected by stimulation frequency but is prolonged by increase in the number of shocks applied. Stimulation of a pedal ring at the onset of a contraction prevents the further development of this contraction, while stimulation as a contraction reaches its maximum is followed by more rapid relaxation than in unstimulated controls. Mid-column rings when freshly prepared show a latency of the order of 120 sec. As the preparation ages, a double response to stimulation appears ; the first response has a latency of about 30-40 sec. and presently becomes the only type of response shown. If two sets of stimuli are applied to a mid-column ring, the magnitude of response to the second set increases as the time between stimulations increases. With long intervals an almost total contraction is obtained in response to a single shock. The effect of intercalated stimuli upon the rhythm of spontaneous activity is studied. The effect is very variable and it is suggested that this is the result of electrical stimulation having both an excitatory and an inhibitory effect. The very long latent periods characteristic of pedal rings and the rhythmic activity of these preparations are interpreted as interactions of excitation and inhibition.

THE ASSOCIATION BETWEEN THE HERMIT CRAB EUPAGURUS BERNHARDUS (L.) AND THE SEA ANEMONE CALLIACTIS PARASITICA (COUCH)

SUMMARY Observations and experiments on the association between Calliactis parasitica and Eupagurus bernhardus have been carried out. The attachment of the anemone to the shell is achieved by a remarkable manoeuvre involving the attachment of the tentacles and oral disc, the freeing of the pedal disc and its adhesion to the shell by a somersaulting movement, with the eventual release of the oral attachment and the assumption of the normal posture on the shell. Active participation of E. bernhardus in detaching the anemone and transplanting it on the shell was not observed, in contrast to the active parts played by the crabs Pagurus striatus and P. arrosor in their associations with C. parasitica. C. parasitica becomes attacvhed to empty shells no less quickly or frequently than to shells occupied by E. bernhardus. The anemone readily settles on shells of living Buccinum undatum in the laboratory and does not desert these for shells occupied by crabs. It does not settle on shells cleaned by boiling in strong alkali. The respective roles of anemone and crab in the associations between Calliactis and these three species of pagurid crabs are discussed from the stand‐points of comparative behaviour and with special reference to the capacities of the actinian neuromuscular system here displayed.

Observations on the Structure of Hydra as seen with the Electron and Light Microscopes

ABSTRACT Sections of hydra studied with the electron microscope show various structures which have been identified by referring to control histological sections and to previous descriptions. Certain features have also been examined in frozen-dried sections under the light microscope. In the ectoderm, epithelio-muscular cells contain various organelles, and also smooth longitudinal muscle-fibres with which mitochondria may be associated. The so-called ‘supporting fibres’ appear to be thin bundles of muscle-fibres. Although points of contact exist between muscle-fibres, there appears to be no cytoplasmic continuity. The muscle-fibres insert on the mesogloea, and appear to be separated from it by two membranes, one belonging to the cytoplasm surrounding the musclefibre and the other to the mesogloea. The mesogloea is extracellular and quite distinct from the intracellular muscle-fibres. It appears granular and sometimes presents an indistinct fibrous background. In frozen-dried material the mesogloea stains blue with Mallory’s method, while the muscle-fibres stain red. Two main types of cells are found in the endoderm. Among these, some of the digestive cells contain transverse muscle-fibres, but they are less distinct than the longitudinal ectodermal fibres. Otherwise the digestive cells vary much in structure, but generally they contain vacuoles and their free surface is thrown into villi covered with small granules. The ‘foamy gland cells’ are filled with much larger vacuoles containing granular material. The vacuoles are discharged together with portions of cytoplasm, and at this stage lamellated double membranes and mitochrondria appear between the vacuoles. Both types of cell possess two flagella, which show a typical ultrastructure and are surrounded by a thick membrane. Various other cells of the ectoderm are distinguished by their characteristic appearance. Cnidoblasts, for instance, have been found to contain an extensive system of intercommunicating vacuoles bounded by membranes, and do not resemble the interstitial cells. In unexploded penetrant nematocysts the tube is preformed and the butt and stylets can also be seen. The special gland-cells of the pedal disk show large, electron-dense granules which are extruded from the cell without any cytoplasm. A relatively thick homogeneous layer on the surface of the pedal disk is distinguished by the electron microscope.

The Organization of the Muscular System of Metridium senile

ABSTRACT The muscular system of Metridium consists of fields of relatively short musclefibres. In extension these may exceed i mm. in length but are only about 0-5/2 thick. They can shorten to about a fifth of the extended length. The fibres consist almost entirely of densely staining material. They form a connected network. At least in some cases the cells seem to be in contact rather than to form syncytial connexions. Deformation of the body-wall is in part controlled by the contractility of the muscle-fibres and in part by the properties of the mesogloea. Longitudinal contraction of the body-wall is accompanied by great thickening of the substance of the mesogloea. That part of the mesogloea which carries the circular muscle-fibres of the body-wall does not thicken. It buckles, thereby throwing the muscular layer into folds. Buckling occurs during the shortening of almost every actinian tissue. The familiar folding seen in cross-sections of the retractors is a special case of excessive buckling which is permanent. A natural limit to the extension of anemone tissue is reached when the musclelayer is completely unbuckled. If contraction proceeds to a maximum, there is a second order of buckling by which the whole body-wall is thrown into folds. Contraction can then proceed no further. The function of the muscle-fields is analysed. The youngest cycles of mesenteries (‘imperfect microcnemes’) supply the longitudinal musculature of the column (parietal muscle). The older ‘imperfect retractor-bearers’ have only feeble parietal musculature, but possess a retractor muscle connecting the oral with the pedal disk. The perfect mesenteries have a similar organization to the imperfect retractor-bearers, and particularly in the non-directive perfect mesenteries there is a well-developed sheet of radial (exocoelic) muscle whose reflex contraction opens the mouth. The vertical endocoelic muscle-fibres of all non-directive mesenteries fan out on to the pedal disk. On the exocoelic side, the parieto-basilar fans out from the pedal disk to the body-wall. As usual, the muscle-fields of the directives are developed on opposite sides from those of the non-directives. The muscular plan of the pedal disk is compared with the tube foot of Asterias as described by J. E. Smith. There is a significant functional similarity in the operation of vertical, oblique, and radial muscles (basilars) bearing on the adhesive disk. The circular layer of the actinian foot has no analogue in the tube foot. It is primarily concerned with locomotion and not with adhesion. The functional organization of the oral disk and tentacles is discussed. It differs from the rest of the body in the retention of ectodermal longitudinal muscle. This layer is responsible for the special movements executed in feeding. The significance of its physiological separation from the endodermal system is noted.

The Mesoglea and Muscle-fibres of Chlorohydra viridissima

ABSTRACT The structure and relations of the mesoglea, muscle-fibres, arid cell layers were examined by several histological techniques. Evidence is given that the muscle-fibres represent specializations of the mesogleal substance, and they are therefore referred to as mesogleal fibres. The longitudinal fibrous system is regularly demonstrable in the body of the animal and the circular system in its tentacles. The circular system in the body cannot regularly be shown and is variable in its appearance. The evidence from the study of the fibres, and of the mesoglea of the pedal disk, leads to the suggestion that some at least of the fibres are labile structures, ‘crystallizing’ from the mesoglea according to the shape and movements of the animal.

DEVELOPMENTAL ANALYSIS OF SCYPHOMEDUSAE

SummaryThe eggs of the scyphomedusae vary in size from 0–03 to 0–3 mm. diameter, a variation of a thousand‐fold in volume. Type of gastrulation is correlated fairly closely with egg size, the smallest eggs gastrulating by unipolar ingression, the largest by complete or partial invagination, while eggs of intermediate size gastrulate by a combined process of ingression and invagination. The nature of the subsequent development is equally correlated with egg size and type of gastrulation. The smallest become vermiform planula larvae incapable of constituting a polyp directly, the intermediate‐sized eggs form planulae which become polyps or scyphistomae, larger eggs may occasionally form actinula larvae and so form a polyp without having had an active planula existence, while the largest eggs develop directly into the ephyra‐medusa, omitting the scyphistoma stage entirely. In the case of the smallest, and of the smaller intermediate types, the blastula and planula stages have the capacity to grow as such without accompanying progress in differentiation. Budding may occur even at the planula stage.The scyphistoma develops four, sometimes six, gastric pouches, and forms tentacles from the rim of the oral disc, usually in the order 2, 4, 8, 12, 16, and by further addition of fours to attain sometimes as many as 32. It consists of a head and a stalk, though either part may be much the longer. Outgrowths from the body wall near the junction of head and stalk may form either pedal stolons for attachment, or buds. Buds may be liberated as free‐swimming pseudo‐planulae, may develop attached to the parent and then liberated, or may develop at the ends of long stolons. Buds of a statoblast function may be formed successively beneath the pedal disc. Tentacles of a fully grown scyphistoma may constrict off, move away as pseudo‐planulae and give rise to new polyps. There are regional differences in the capacity of pieces of the body wall to regenerate whole or partial polyps. Isolated endoderm has no capacity to reconstitute a polyp. Isolated ectoderm from any level can reconstitute a perfect whole.The type of strobilation depends upon the size and shape of the scyphistoma head or column. When the head is shallow and the stalk long, strobilation is usually monodisc, resulting in one, sometimes two ephyrae. When the head is large and more or less columnar, segmental constrictions appear, and strobilation is typically poly‐disc. Isolated segments of a polydisc strobila do not necessarily develop into ephyrae, but may reconstitute themselves into scyphistomae and become reattached as typical polyps. In polydisc forms at least, the initiation of strobilation depends upon the attainment of a large size, accumulation of food reserves, and a certain critical low temperature. Most ephyrae are liberated with eight lappets, but the first ephyra formed, that is, from the oral disc, usually has more than eight, while the last to be formed may have smaller numbers. The initial number, with their corresponding rhopalia, persists as the final number in the mature medusa.

Contributions to the Comparative Anatomy of Some British Actiniæ

The main object of this paper is to give a comparative anatomical description of some British Anemones as well as to describe each individually; but the account is not so full as it was expected to be, as it was not possible to get all the species required.In most cases the specimens were examined externally before sections were cut. Each specimen was prepared according to the usual method used with paraffin wax, and I found during the progress of the work that borax carmine was the stain which gave the most satisfactory results.I should like to thank Miss E. de Fraine, D.Sc., Mr. F. W. Durlacher, and Mr. F. S. Wright for much assistance in the course of the work.For specimens collected and sent I am deeply indebted to Dr. E. J. Allen and his staff at Plymouth, Miss Delap, of Valentia Island, and Monsieur Louis Fage, of the Laboratoire Arago, Banyuls-sur-Mer, France.FAM. SAGARTIDÆ (Gosse, 1858).“Actiniinæ with a contractile pedal disk; body-wall smooth, or provided with verrucæ or tubercles, and usually perforated with cinclides, with or without a cuticle. Tentacles usually numerous and retractile, not very long, smooth, simple and generally entacmæous, sphincter muscle characteristically well developed and mesogloeal, occasionally diffuse endodermal, or even absent. At least six pairs of mesenteries, may be fertile or sterile. Acontia present.” The above is the definition used by Haddon in “The Actiniaria of the Torres Straits” in 1898. There has been a great deal of discussion concerning this family.

Kodioides borleyi, n.sp

In his Report on the Actiniaria of the Norwegian North-Atlantic Expedition, D. C. Danielssen erected the genus Kodioides to receive the remarkable form which he named K. pedunculata, a single specimen of which was dredged at Station 35, July 5th, 1876, in 1050 fathoms, Lat. 63° 17′ N., Long. 1° 27′ W., the bottom being Biloculina Clay. The detailed description will be found in the section “Actinida,” Vol. V, pp. 77–82, PI. VI, Tigs. 3–4; XXII, 8–11; and XXIII, 1–4. The genus is there characterized as follows: “The body encrusted, piriform, with a long bare stem terminating in a pedal disc. Two series, containing a few retractile tentacles. 12 pairs of septa, of which 6 pairs perfect. Suckers on the encrusted portion of the body. No gullet-groove. Mesodermal, annular muscles. Acontia.”

Regeneration in the anemone, Sagartia luciae

1) Tentacles regenerate at the oral end of aboral pieces cut at different levels; even on very short pieces. 2) The absence of the pedal disk in a piece does not effect the regeneration of tentacles, and the time required for the formation of a new disk is longer than that for new tentacles. 3) A cut in the column or in the oral ring does not cause a growth of tentacles unless, in the latter case, some hare been lost. If tentacles have been removed, new ones generally regenerate at the place of the cut, but they may be intercalated at intervals around the oral ring. 4) A pedal disk regenerates only at the aboral end of a piece, and tentacles only on the oral end of pieces (with the exception of a few heteromorphic tentacles on very small oral pieces). 5) Neither gravity nor contact effect the regeneration of tentacles, or the orientation of the individual. 6) The apparent change of orientation is only found in very small pieces, which, however, by means of rapid growth at the place of the cut ultimately assume the same orientation at that of the individuals from which they were cut, thus showing that even the smallest pieces have a definite polarity.

The regeneration of an oesophagus in the anemone, Sagartia luciae

1) An oesophagus regenerates in small pieces cut from the base of Sagartia luciae as an invagination of the mesogloea and endoderm in the shape of an inverted cup in which the mesogloea forms the middle layer and is covered on both outside and inside by endoderm. Later the ectoderm breaks above the cup and becomes continuous with the endoderm lining it. The ectoderm takes no part in the regeneration of the oesophagus. 2) Tentacles develop as outgrowths of the old body wall (including ectoderm, mesogloea and endoderm). 3) New mesenteries regenerate as infoldings of the mesogloea and the endoderm which become attached to the oesophagus. 4) The orientation of the regenerating piece may, or may not, be the same as that of the individual from which it was cut. 5) The fixation of the piece to the bottom of the dish takes place before the oesophagus regenerates. The contact, in some ways, stimulates the regeneration of an oesophagus at the opposite end. If the piece is cut so that it contains a small part of the old pedal disk or if it has lost the power of turning itself over, it must remain in the position in which it falls and its orientation is therefore determined by gravity.

On the nervous system of Actinia .—Part I

MM. Milne-Edwards and Jules Haime wrote as follows in 1857 concerning the nervous attributes of the group of Cœlenterata called Zoantharia :—“ They (les Coralliaires) enjoy a highly developed sensibility ; not only do they contract forcibly upon the slightest touch, but they are, moreover, not insensible to the influence of light. Nevertheless, neither a nervous system nor organs of special sense have been discovered in them. It is true that Spix described and figured ganglions and nervous cords in the pedal disk of Actiniæ ; but the observations of this naturalist, so far as polypes are concerned, are not entitled to the least confidence. " Some naturalists have supposed that the ‘ bourses calicinales ’ of the Actiniæ are eyes, and M. Huschke believes that certain capsules in the trunk of Veretila , which contain calcareous bodies, are the organs of hearing. But these hypotheses do not rest upon any proved facts.”