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Seventh Report on Earthquakes in Scotland, drawn up by Dr. BRYCE, · F.G.S., F.R.S.E. The Committee consists of Dr. BRYCE, F.G.S., Sir
W. THOMSON, F.R.S., J. Brough, G. FORBES, F.R.S.E., D. MILNEHOME, F.R.S.E., and P. DRUMMOND.
The state of quiescence alluded to in last year's Report has suffered scarcely any interruption during the current year. No movement has occurred of sufficient intensity to affect any of the instruments employed by the Committee for testing the shocks. The Association will be aware that these are the seismometer, constructed on the principle of the inverted pendulum, which is placed in the tower of the parish church of Comrie, and two sets of upright cylinders, described in last year's Report, which stand on boards on the sanded floor of a building erected two years ago by the Association upon a site, half a mile west of the Comrie church, kindly granted by P. Drummond, Esq., of Dunearn, in the grounds surrounding his house. This building stands in the Comrie valley, on a boss of rock of the samo kind of slate of which the adjacent hills and ridges aro composed, and which can be traced into continuity with those on both sides of the valley. It was therefore expected that cylinders so placed would readily respond to any movement affecting the rocks on either side of the valley, more especially as the centre or focus from which it has hitherto been considered that the movements have emanated is at no great distance on the north side of the valley.
This expectation has not been realized, inasmuch as two slight shocks were experienced on the 14th and 16th of January, in the morning and afternoon, without affecting the seismometer or the cylinders, even those of smallest diameter, which a very slight movement is sufficient to lay prostrate in the sand. It is easy to see that a very extreme sensibility must be avoided in order to guard against the effects of other disturbing causes as a storm of wind, a peal of thunder near at hand, or a heavy footfall on the rock outside; and hence that an undulation, propagated from a distant centre, might be so retarded by the resistance of rocky masses as not to produce the required amount of disturbance. The evidence furnished by several most intelligent and trustworthy persons leaves no doubt that on the day mentioned a very slight shock was really felt on the north side of the valley; that the movement seemed to come from the westward, and was attended by a slight noise, which died gradually away towards the south-east.
This somewhat disappointing result has led your Committee to add two more cylinders of increased delicacy to each set, and to use every effort to obtain suitable sites for other sets more to the west and north, and also further down the valley, as near Dunira, the conjoctured focus, and that fixed on by Mr. Milne-Homo in the former inquiry, in Glen Lednoch near the edge of the eruptive granite tract, whence the late disturbance seems to have proceeded; and, if possible, also at Ardoch, Dunblane, and Bridge of Allan, at all of which the shocks of 1873 were so severely felt. The expense would be inconsiderable; the difficulty to be encountered is the procuring of a suitable and safe site and a competent observer. Your Committee earnestly hope that these obstacles will be overcome in the course of the succeeding year.
Report on the Present State of our Knowledge of the Crustacea.
Part II, On the Homologies of the Dermal Skeleton (continued). By C. SPENCE BATE, F.R.S. 8c.
[Plates II., III.] As in the first part of this Report the carapace or dorsal surface of the Crustacea was considered, it is now intended to examine the plastron or ventral surface, and so complete our inquiry into the form and structure of the dermal skeleton, previous to a consideration of the internal viscera and development of the animals of the various forms in the class.
The head, or cephalon, is more clearly defined in Edriophthalmous Crustacea than in any other order ; but even here the somites posterior to the mandi, bular ring have the dorsal surface wanting ; but a clearly defined character distinctly separates them from the somites that pertain to the succeeding seven, which constitute the pereion.
This condition is less complete in Squilla (which M. Milne-Edwards has selected as being “ of all Crustacea that in which the 21 segments of the body are the most distinct”), where the posterior somites of the cephalon as well as the anterior two of the percion are only represented by their ventral sur, faces.
This apparent incompleteness of structure, which is due rather to an economy of material, has led carcinologists to consider generally that the cephalon and pereion should be treated anatomically as one portion of the animal under the general name of cephalothorax.
Thus Dana, in writing on the “ Classification of Crustacea,” in his · Report on Crustacea of the United States Exploring Expedition under Capt. Chas. Wilkes, U.S.N.,' p. 1397, says, “ In these highest species, nine segments and nine pairs of appendages out of the fourteen cephalothoracic belong to the senses and mouth, and only five pairs are for locomotion."
This he has taken from the Brachyural or Macrural decapod, as being the highest types of the order ; but if we are to report our experiences and define the names and conditions of things according as they are represented in a single type or group, every student of any special form will draw his own conclusions from that which he has alone closely considered, and the study of Crustacea as a class in the animal kingdom must be retarded, if not misrepresented.
In studying scientifically the Crustacea as a whole, it will be found not only more correct but more convenient to describe and name the several parts of the animal by their homologous certainty rather than by their adaptation to fulfil different functions which demand a variation of form with the greater or less importance of their requirements.
The seven somites that form the cephalon are most closely associated, and difficult to be separated from those that follow, in the Brachyural type. This circumstance appears to be largely due to the powerful character of the mandibular appendages. The great strength of these organs requires such an internal development of parts that they appear to preclude the posterior somites from the power of growth ; consequently they become merely sufficient to support appendages of a supplementary character.
This is very apparent in the Macrural order. In Palinurus the mandibles are so broad and large that their removal is almost a complete decapitation. It is therefore a structural necessity that the posterior two somites of the cephalon should be supported by those to which they are most closely
approximate ; consequently they are frequently found fused with the anterior somites of the pereion.
Yet in this very genus, in a young state, we have the most complete evidence of the limits that define the cephalon from the pereion, and this again from the pleon.
In the larva of Palinurus, as well as in the animal known as Phyllosoma, which is now generally accepted as being the young of Palinurus after somo weeks' growth, the cephalon is seen to coincide with the limits of the carapace and terminates anteriorly to the seven somites of the pereion. It therefore appears that it is desirable to identify these first soven somites as belonging to the head or cephalon and that only.
The pereion, or thorax, is also composed of seven somites or segments; and this number is never departed from, even in the most depauperized condition of the animal. These several somites Prof. Milne-Edwards, in his “ Observations sur le Squelette tégumentaire des Crustacés décapodes, et sur la Morphologie de ces animaux," Ann. des Sciences Nat. p. 268, 1854, says:« In order to determine easily each of these anatomical elements of the integumentary skeleton, it is desirable to define them by a name; and I shall call them protosomite, deutosomite, mesosomite, or tritosomite, tetartosomite, pemptosomite, hectosomite, and hebdosomité, following the order which they occupy from before to behind.”
In the lower types they form, as in the Amphipoda, separate and distinct segments; but in the higher groups, as we see the dorsal surface of the somites of the cephalon developed and produced posteriorly so as to cover and protect the upper part of the pereion, so we find the somites of this latter division coalesce ventrally more or less perfectly until in the Macrura and Brachyura they reach the highest degree of consolidation and are much more dense and strong than is the structure of the carapace.
This condition is gradually seen to be approached through different stages from the Edriophthalmia upwards. In the genus Squilla (which has many analogies with the sessile-eyed Crustacea, and appears like an enormous stalk-eyed Amphipod) three or four of the posterior somites are exposed beyond the carapace and have the dorsal arc complete and separately perfect. In the Diastylidæ we see the same ; and ultimately in the genus Pagurus, among the Anomurous Crustacea, there is but a single somite that is not embraced within the limits of the carapace, and that is reduced to a very slender ring.
With the deterioration of the dorsal arc of each somite of the pereion the ventral arc increases in density and coalesces the more perfectly with its neighbours. This appears much to depend upon the habits and character of the animal. If it be one whose habits are perambulatory, as in Palinurus, the somites are strongly fused together into a strong broad sternum; whereas in such animals as Palamon and Homarus the sternum is less strongly developed, and apparently of a more feeble character.
This depreciation of the sternum gradually goes on as we approximate the short-tailed orders, and arises from the absorption of the first joint or coxa of the leg into the general system of the animal.
In Palinurus the sternum (Pl. II. fig. 1), corresponding to the posterior five somites, is very broad, and the legs are very widely separated from those on the opposite side ; in Homarus, Nephrops, and Astacus (Pl. II. fig. 2) they approximate each other so nearly that the sternum consists of a small calcareous longitudinal cord, to which the apodema are attached and receive their support.
In the Aņomura, of which we may take Lithodes (Pl. II. fig. 3) as an example, the coxæ of the legs are so closely compressed together laterally that, without coalescing or being fused together, they are apparently united, while the inferior part of each coxa is completely fused with its neighbour for about half its extent.
This is carried still further in the true Brachyura (Pl. II. fig. 4), where the first joints of the legs are all consolidated into a tolerably perfect mass of calcareous structure, and resemble the nature and character of a sternum,
The ventral plastron, therefore, is formed of the first joint of the leg, and the inferior arc of these seven somites is wanting in the true Brachyura in the adult stage, the inferior surface of the legs fulfilling the duty of the sternal plate. As I have already observed, this state can be traced gradually from the Macrura to the Brachyura ; and it may also be observed gradually to assume this condition by following the development of the young, in which the coxal joints may be distinguished separate and individually present, and gradually coalescing as the animal increases in dimensions with age. I am aware that this assertion is not in accordance with the teachings of previous carcinological anatomists; but it is one that can be proved to demonstration.
Milne-Edwards, “ Observations sur le Squelette tégumentaire des Crustacés décapodes,” Ann. des Sc. Nat. p. 269, 1854, says, “ These rings exhibit all the tergal pieces, and are closed above by a carapace, except among a small number of Anomura, as the Cenobitis, where the seventh ring is complete. We can distinguish always a ventral arc, constituted normally by two sternal and two episternal pieces, and a dorsal arc, represented upon the sides of the epimeral pieces of the sclerodermic prolongations extending between the ventral and dorsal arcs of each ring, so as to enclose between them each side of the body, and to circumscribe before and behind the articular cavities destined for the insertion of the corresponding members. When the rings are free, each of these arcs' extremities I shall call arthrodials, for the sake of being distinct; but when the zones are soldered together it is different. The anterior arthrodial of each thoracic ring is united to the posterior arthrodial of the preceding zone, and is more or less completely united with it, so that the interarticular space situated between two such legs, instead of presenting two sclerodermic rings, lodges only a single arthrodial prolongation, which becomes common to the two approximating frames, so that it appears to depend more especially upon the last of the two rings so united. To simplify the description, I shall consider these complex arthrodials as if they were formed only by their most important parts, and shall neglect consequently their anterior plate; but it should be observed that we can nearly always recognize its existence. There is also an interannular symphysis which results from the formation of an interior fold of the sclerodermic lamella, a fold the two plates intimately sustain between them. These processes must be looked upon as if they were produced by the simple lamella of the posterior border of one of the segments so united by symphysis.
" It is always in the anterior portion of the thorax of the decapods that consolidation of the integumentary skeleton is carried to the furthest limit by the soldering or fusion of the anatomical elements.”
Now what I contend is, that the structure of the somite has, as a part of the dermal skeleton, ventrally disappeared in the Brachyura, and its place has been taken by the dermal tissues of the first joint of the several legs of the pereion, and the apodema is formed in the various families of Crustacea out of parts that are homologically distinct.
In the Anomura, of which Lithodes may form the best example, the coxæ may best be dissected out; and it does not require any very extreme care to separate the frame of one appendage from those by which it is compressed both anteriorly and posteriorly, by which compression the joint partakes of a quadrilateral form. The plates are in many places reduced to an extreme tenuity, and practically fulfil the office of a single wall, although in reality they are produced by two lamellæ closely compressed but not united. The inferior or ventral wall, that forms the sternum, is very much more strong, and extends until it meets the corresponding plate upon the opposite side. In Lithodes this simple condition extends from the anterior to the posterior extremity of the pereion.
In the Brachyura, of which we may take Cancer as the type, the walls of the coxal joint form the floor of the pereion from the anterior extremity to the fourth or tetartosomito, from which posteriorly an upright wall in the median line separates the right side from the left, and encloses the muscles of the four posterior pereiopoda within as many corresponding chambers, forming a strong arch that supports the internal viscera and precludes their sinking into the ventral cavity.
If, as I contend, this condition of the structure may be demonstrated beyond doubt, it follows that the episternal pieces lose their homological signification, as defined by Prof. Milne. Edwards, in the same way as the epimera of the dorsal aro.
The episternal plates are parts of the first or coxal joint of the legs produced as plates, valuable as supporting the articulations of the next succeeding joint with the first. It is interesting to observe that these so-called episternal plates can be traced back to large spinal processes in the young animal, and to less important processes in the pupal or third stage in the process of the development, where they can be distinctly seen as parts of the coxæ of the appendages attached to the pereion (fig. 7).
This appears to be the anatomical condition in the Brachyura, and also in somo of the Anomural groups.
But in the Macrural type the ventral surface of the pereion is formed of the lower arc of the several somites which belong to this division of the animal. Some slight variations of form and appearance exist in separate genera. In Palinurus the anterior part of the sternum is narrow and longitudinally longer than broad, while the posterior part gradually increases in width from the anterior to the posterior extremity. Each somite is completely fused with those with which it is in contact at the centre, while deep lines of fissure define their separation on each side, the posterior process of which somites corresponds analogically with the so-called cpisternal plates in the Brachyura, but homologically they are distinct, being, in this form, parts of the true somite, and not a portion of the coxa of the leg incorporated with it (Pl. II. fig. 1).
In the genus Astacus the sternal plates are all narrow, being scarcely broader posteriorly than they are anteriorly, while in the genus Homarus the sternal plates are still more narrow and less important. This appears to be the general characteristic of the rentral plates in Nephrops, Palamon, Crangon, &c., but more delicately and feebly constructed, so far as the external conditions; but in the lower forms of Crustacea, such as the Amphipoda and the Isopoda, the sternal plates are broader than they are long, and consequently the several pairs of appendages are widely separated from each other, correspondingly so throughout the entire length of the pereion.
The internal structure in the Podophthalmous types is more complex than the same parts in the lower or sessile-eyed forms.