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REPORT ON THE MARINE INVERTEBRATE FAUNA NEAR
DUBLIN. By G. Y. DIXON & A. F. DIXON.
[Read APRIL 27, 1891.) Ix 1890 the Academy gave us a grant to investigate the Marine Invertebrate Fauna in the neighbourhood of Dublin, and to make observations on such forms as we could succeed in keeping alive.
We now present a report of such observations as we have made.
This anemone is very plentiful at Dalkey Island, hanging in profusion from the under-sides of the loosely-piled granite rocks at the western end. The orange variety is most common, but very fine specimens of Gosse's var. sindonea are also abundant. We have found this species also at Monkstown and on the North Bull Wall, but the specimens were not so fine or so numerous at these localities. Gosse (Actin. Brit. p. 13) says the lip is always rufous, or orange red, whatever the hue of the body. No white specimens which we found had a red or orange lip, this part being, so far as we have observed, always of the same colour as the rest of the animal. We call attention to this, not because we doubt the occurrence of such individuals as Gosse describes, but because of the importance attached by Gosse to the presence of the thick orange-coloured substance found by him in the region of the lip inside the wall of the esophagus, and supposed by him to be a liver. (Actin. Brit. p. xvii).
There is no question that the number of esophageal grooves in this anemone is not constant; though most individuals have but one groove, still it is by no means unusual to find a second present. This fact was pointed out so long ago as 1861, by Mr. F. J. Foot, in a Paper read by him before the Dublin Natural History Society,' yet it does not seem to be recognized in subsequent accounts of the species. No
1 Proc. Nat. Hist. Soc. Dub., vol. iž., p. 63. It should be observed, too, that in his list of errata on p. 362, Gosse (loc. cit.) adds "the qualifying phrase 'in general' to the character that there is but a single mouth-angle and pair of tubercles."
doubt Mr. Foot describes only one specimen as having two grooves, and, as he immediately goes on to mention some instances of peculiar monstrosities, perhaps readers of his paper were led to believe that the occurrence of a second oesophageal groove at the opposite end of the mouth to the first groove was also a sport or freak. We have, however, found it to be present far too frequently to justify any such conclusion. We may add also that we found one otherwise normal specimen with three grooves.
Not unfrequently this anemone exhibits the curious mode of reproduction by fission, described by Gosse (Actin. Brit. p. 19: cf. Van Beneden, Recherches sur la Faune littorale de Belgique, pp. 189, 192). As the animal moves from one spot to another it often leaves behind small fragments of the margin of the base, which continue to adhere and develop into full-grown individuals. It would seem as if the parts that thus separate from the parent are not previously in any way specialized, but only remain behind because their adhesion is so strong that the animal finds it easier to tear its own tissues than to loosen their hold. But we have also observed cases in which, before the fission occurred, the flat expanded portion of the body-wall showed signs as though something like a bud were being developed just on the margin of the pedal disk. In such cases a slight nodular thickening of the expanded portion of the body-wall takes place, and an appearance is presented not unlike the slight rise of the cenenchyma, which in the encrusting forms of the Zoantheæ precedes the growth of a fresh polype; then when the parent moves, these nodular thickenings are left behind, enter on a separate existence, and develop a column, disk, and tentacles of their own. It would appear, however, that these buds do not always part company with the parent animal, at least at an early stage of growth, for we have met with a good number of specimens where a small animal was united to a large one by a common base, presenting all the appearance of a parent and a bud which had continued to grow and thrive together. Gosse has remarked the frequent occurrence in this species of the monstrosity of two disks springing from a single column. Foot records the occurrence of a specimen with two mouths on one disk. We have met with one monstrosity like that described by Foot, and two or three such as Gosse mentions. We may add that one of these specimens with two disks, had one mouth with one groove, and the other mouth with three grooves.
We have examined three very small specimens by means of transverse sections. One of them had two @sophageal grooves, and as
might be expected its transverse sections exhibited two pairs of directive mesenteries, a pair corresponding in position with each groove. There were four other pairs of perfect mesenteries, and the arrangement of the perfect mesenteries was exactly the same as that which characterizes the adult Halcampa chrysanthellum. The arrangement of the imperfect mesenteries is not so easy to describe, as they were by no means regularly developed in the exocæles. For the purposes of description we will imagine one pair of directives to be set at the top, and the other pair at the bottom of the transverse section, and beginning at the top we will enumerate and describe the mesenteries as they occur on the right hand side of the section when it is so placed. The first exocole contains but one imperfect mesentery, which is pretty well developed, and has a very considerable filament. Then come the two pairs of perfect lateral mesenteries, there being no imperfect mesenteries developed in the exocole between them. In the next exocæle, which lies between the second pair of perfect lateral mesenteries, and the directives at the bottom of the section, there is a very small pair of imperfect mesenteries, which at no portion of their entire height present a filament of any length. On the right side of the section then we have but three imperfect mesenteries—one in the first exocole, none in the second, and two in the third. Now, let us see how the left side of the section is furnished. A glance shows that it is regular, that is, there are two pairs of perfect lateral mesenteries, and a pair of imperfect mesenteries occupies each of the three exocæles, making six imperfect mesenteries on this side. The total number of imperfect mesenteries therefore is nine, which, added to the twelve perfect, gives us twentyone in all.
In each of the two remaining specimens there was but a single csophageal groove; and when we cut the sections we found there was but one pair of directive mesenteries. In one of these specimens with the single groove there were twelve perfect mesenteries arranged in the same way as the perfect mesenteries in Halcampa chrysanthellum, except that instead of the second pair of directives there was an ordinary pair of mesenteries with the longitudinal muscles developed on their adjacent sides. The imperfect mesenteries were symmetrically arranged in the following manner :-In each of the exocæles adjoining the directives there were three pairs of imperfect mesenteries, two being small pairs situated on either side of a larger pair. An exactly similar number and arrangement were present in the two next the lateral exocæles, while in the two remaining exocæles—those
R.I.A. PROC., SER. III., VOL. II.
furthest from the directives—there was but a solitary pair of imperfect mesenteries. To sum up, then, there were in this specimen twelve perfect and twenty-eight imperfect mesenteries—forty in all.
In describing the arrangement exhibited by the remaining specimen, we will suppose the transverse section so placed as to have the single pair of directives at the top; and as before, will describe the mesenteries as they occurred, starting from the directives and proceeding downwards on the right side. In the first exoccele there was a single pair of imperfect mesenteries ; then came the first perfect pair: the second exocole contained three imperfect pairs, two small and one large pair between them; then came the second perfect pair, and next in the third exocæle a single pair of imperfect mesenteries; then, exactly opposite to the directives, the third pair of perfect mesenteries; and in the fourth exocæle another single imperfect pair. Next in order one would have expected to find the fourth pair of perfect mesenteries, but the section here presented only one perfect mesentery, and corresponding to it an imperfect one, the two—as the longitudinal muscles were adjacent constituting a pair, one of which, that nearest the directives, fell short of the esophagus. The fifth exocæle contained no imperfect mesenteries, so that the fifth pair of perfect mesenteries came immediately next the anomalous pair just described ; and finally, in the exocele separating the fifth pair from the directives was a single pair of imperfect mesenteries. The total number in this specimen, then, was twenty-six, of which eleven were perfect.
Besides the information respecting the mesenteries derived from transverse sections, we have also to record some observations made in the case of a very transparent specimen, which lived for a long time in our possession. This specimen attached itself by the base to the glass front of the tank, so that the lines of the insertions of the mesenteries were very easily counted. Its base showed ten pairs of lines running into the centre from the edge, and between these pairs, ten pairs that did not reach quite so far as the centre, twenty pairs that did not reach more than half way, and forty very minute pairs that only ran in a very short way from the edge. The ten secondary pairs alternated with the ten primary, and the twenty tertiary were developed in the spaces between the primary and the secondary, while the forty minute pairs were distributed one in each space, lying between the larger pairs, the whole arrangement being perfectly symmetrical. This specimen had but a single groove. The transparency of the specimen we have just mentioned enabled us to observe in its mesen
ibuted orent being he transparen it's me
teries the presence of the outer series of stomata, which have been described and figured by both Gosse (Actin. Brit., p. 19, Pl. xi. fig. 1) and Hertwig (Actinien, p. 63, Pl. ii. fig. 1).
This species sometimes exhibits the phenomenon of a single tentacle, being greatly elongated and arching over the other tentacles. Though this tentacle may continue thus extended for some days, it ultimately returns to its normal condition, when it is not distinguishable from the others. The tentacle so elongated does not seem to bear any special relation to the angle of the mouth.
In this species we have found bifurcated tentacles occurring occasionally.
One large Metridium dianthus in our possession while roving through the tank stumbled across a fine specimen of Bunodes verrucosa. It poured out an enormous number of acontia over its less aggressive companion with such effect that the Bunodes verrucosa never recovered from the attack. Though it had been perfectly established in the tank for several months, and was almost always fully expanded, it at once closed itself, and kept sloughing its epithelium continually, and drooping more and more till it died.
This species, like Actinia equina, has the power of floating on the surface of the water, base upwards; the base being slightly depressed in the centre and quite dry. We have observed this habit in springtime only, and in the case of specimens that had been long domesticated in the tank. It is interesting to watch an individual proceeding to float. Having climbed to the surface, it slowly relaxes its hold of the side of the tank, and bends its pedal disk outwards along the top of the water, keeping the centre of the pedal disk concave. Though the animal has been hitherto quite submerged, the pedal disk, as it thus turns outward along the surface, is seen to be absolutely dry, this condition being due to the extremely close contact which it maintains with the substance to which it adheres.
We have found this anemone at Malahide. It had previously been recorded for the county Dublin, having been obtained by A. H. Hassall at Dalkey (An. and Mag. Nat. Hist., vii., p. 285). Every specimen taken at Malahide was imbedded in mud, its pedal disk being attached to a small stone two or three inches below the surface of the mud, from which the tentacles and oral disk just emerged. We did not find any of the anemones, as Gosse did at Weymouth (Actin.