Plectrum

“ Unlike the larva [186] used by Ammophila, with its chain of ganglia, in the Araneidæ the whole central nervous system, including the brain and the ventral cord, forms a single mass, pierced by the œsophagus. The greater part of this mass, which lies behind the œsophagus, represents the fused ventral cord from which the nerves radiate. It is evident that a thrust given in almost any part of the ventral face of the cephalothorax, or even on either side of the anterior half of its edges, would reach the nervous centre. ”

“ In some cases figures of the foot show conspicuously the long transverse slit-like opening of an anterior pedal gland, whence the mucus of the thread might presumably be derived. In many Pectinibranchs, however, in addition to the anterior gland, a ventral pedal pore exists in the median line of the anterior half of the foot-sole; it forms the opening of a cavity said to be comparable to the byssal-cavity of bivalves, and from it, externally, a well-marked groove often runs to the tip of the tail. ”

“ In Scalpellum, I ascertained that the new membrane, connecting together the newly-formed calcified rims under the valves of the capitulum, was formed as a fold, with the articulated spines which it bears, all adpressed in certain definite directions. This fold of new membrane, when the old membrane splits and yields, of course expands, and thus the size of the capitulum is increased. In the peduncle, lines of splitting can seldom be perceived, except, indeed, in the sub-globular, embedded, downward-growing peduncle of Anelasma, as described under that genus. ”

“ The ligament is altogether internal; it is without ears, and the prolonged beak so conspicuous in that genus. The Plicatula has the faculty of affixing itself to another body, so that many are found grouped together in clusters. The valves are strongly plaited within and without, closely interlocking with each other.
Shell solid, adhering, sub-irregular, without ears, inequivalve, pointed at the summit, rounded and plaited behind; hinge with two strong teeth in each valve, with a cavity between them, in which the ligament is internally inserted. ”

“ The true Perches (Percina) have two distinct dorsal fins, with the membrane which connects the rays semi-transparent and nearly colourless. The pectorals and ventrals are obtuse, or somewhat rounded; the former contain each five soft rays; the latter are placed beneath the pectorals. The form of the body is oblong; the scales are comparatively large; the mouth is wide, and furnished with short and small teeth much crowded, without any larger pointed teeth, resembling canines, at the sides. ”

“ There is no trace, at this stage at any rate, of a medullary groove; and as, we shall afterwards shew, the central canal of the cerebro-spinal cord is formed in the middle of the solid keel. The shape of this keel varies according to the region of the body. In the head (Plate 35, fig. 18, m.c.) , it is very prominent, and forming, as it does, the major part of the axial tissue of the body, impresses its own shape on the other parts of the head and gives rise to a marked ridge on the surface of the head directed towards the yolk. ”

“ There is no nerve plexus. Each of the cephalic slits generally opens by a pore situated in the centre of a transverse groove, which is beset along one side by a row of shorter grooves at right angles to it. The apparatus consists of a ciliated duct surrounded by nerve tissue, and passing into lobes of tissue which are connected with the brain by thick nerve-cords. The mouth opens rather far forward in front of the brain. The intestinal pouches are symmetrically arranged. Auditory organs are said to exist in some forms, consisting of vesicles containing otoliths. ”

“ Protovertebræ. Each protovertebra [203] or vertebral segment of the mesoblast plate forms a flattened rectangular body, ventrally continuous with the parietal plate of mesoblast. During stage G the dorsal edge of the protovertebræ is throughout on about a level with the ventral third of the spinal cord. Each vertebral plate is composed of two layers, a somatic and a splanchnic, and encloses the already-mentioned section of the body-cavity. The cells of both layers of the plate are columnar, and each consists of a very large nucleus, invested by a delicate layer of protoplasm. ”

“ But, since we know positively that some of its plexuses, as the cardiac, are centres of local stimulation and coördination, which can act independently, though they are influenced by higher centres, it is fairly to be inferred that the other and still larger plexuses, distributed among the viscera, are also such local and largely independent centres; especially as the nerves they send into the viscera, to join the many subordinate ganglia distributed through them, greatly exceed in quantity the cerebro-spinal fibres ac­com­pa­ny­ing them. ”

“ Such a nervous plexus admits of being detected in the sub-umbrella of all Medusæ, and in some species may be traced also into the tentacles. It invariably lies between the layer of muscle-fibre and that of the epithelium. The processes of neighbouring ganglion-cells in the plexus either coalesce or dwindle in their course to small fibres: at the margin of the umbrella these 20 unite themselves with the elements of the nerve-rings. There are also described several peculiar tissue elements, such as, in the umbrella, nerve-fibres which probably stand in connection with epithelium-cells ”

“ In the majority of Plectoidea the spines are rather regularly verticillate, bearing an increasing number of verticils, each of which is composed of three divergent branches. These arise from the three edges of the spine, and all the branches of one edge are usually parallel, either perpendicular to the spine, or directed at an acute angle towards its apex. When the verticils are numerous (five to ten or more) , their size commonly tapers gradually towards the apex. Pinnate spines occur more rarely than verticillate ones ”

“ In juveniles (Fig. 20) the pectoroabdominal seam contains no ligamentous tissue and is like the other interlaminal seams of the plastron. A lateral apex of the pectoral lamina projects upward behind the axillary scale on each side, in the position occupied by the apical scale of adults. Examination of a large series of specimens revealed that the apical scale of adults becomes separated from the lateral apex of the pectoral lamina at approximately the time when the hinge becomes functional as such. ”

“ We will recall at the outset that the lesser pectoral muscle in man is completely covered by the great. In animals this is not the case; the lesser pectoral being very highly developed, projects beyond the great pectoral posteriorly, and occupies to a greater or less extent the inferior surface of the abdomen.
It also consists of two parts: one anterior, which we designate by the name of sterno-prescapular; the other, posterior, bearing that of sterno-humeral. ”

“ Nearly half of the species have it divided into two, a spinous and a flexible one; a large portion of the remainder have the division indicated by a depression in the margin, or a cleft more or less deep, though the membrane is continuous. In some cases, as already intimated, the first dorsal is represented by a few detached spines, either quite destitute of membrane, or each furnished with its own. ”

“ We watched them both, and saw them bring up load after load of pith. They bit out the pellets with their mandibles, and passed them back between the legs and under the body until a quantity had accumulated above the tip of the abdomen. They then walked backward up the stem, and thus pushed out the mass as they came to the top. Often they used the hind legs to assist in getting it out of the way, sometimes kicking it to a little distance. Once in every two or three trips they would come out far enough to expose part of the thorax. ”

“ Here they join to form a simple brachial plexus, from which arise branches proceeding to the musculature of the dorsal and ventral faces of the fin. The seventh to eleventh spinal nerves pass downward to the level of the fin, and then branch, one portion entering the muscles of the ventral body wall, while the other passes into the depressor muscles of the fin.
The pelvic fin is innervated by eight or nine spinal nerves which pass backward and downward along the medial edge of the septum between the myomeres, entering the dorsal side of the fin along its axis. No plexus is formed. ”

“ A little sand at this stage is useful to rub the fingers in, to prevent them slipping, when pulling out small pieces of flesh. The pectoral fins are, of all, the most awkwardly situated to skin out, coming, as they do, at the bottom of the pectoral process, and they must be left with their bones attached, and the flesh be scraped away from between the two fins, and underneath each one, until nothing remains. ”

“ The Solar Plexus.—The veins of this plexus discharge themselves in two directions: (1) By a central canal or canals running along the bottom of the lateral lacunæ of the plantar cushion to gain the deep layer of the coronary plexus. (2) By the Circumflex or Peripheral Vein of the Toe, a canal formed by ramifications from the solar and the podophyllous plexuses, and following the direction of the artery of the same name. The circumflex vein terminates by forwarding branches to concur in the formation of the superficial coronary plexus. ”

“ Sub-Order 12. Pediculati.
Air-bladder without open duct. Opercle large, hidden under the skin; supraoccipital in contact with the frontals, separating the parietals. Pectoral arch suspended from the skull; no mesocoracoid. No ribs, no epipleurals. Ventral fins jugular. Gill-opening reduced to a foramen situated in or near the axil, more or less posterior to the base of the pectoral. Body naked or covered with spines or bony tubercles. ”

“ The plexus pedundus is formed from the spinal nerves coming out of the plexus ischiadicus. These fibers emerge caudalward and are directed horizontally. They frequently anastomose with each other, especially on the pubic rim and on the outside of the plexus ischiadicus. These branches are deeply imbedded in the kidney substance and innervate the pubio-coccygeus, or depressor coccygis lateralis, the ilio-coccygeus, the transversalis, the sphincter, and other muscles of this region, and the skin of the anal region. ”

“ I do not understand how the splitting of the old membrane is effected; but no doubt it is by the same process by which the membrane of the capitulum in other genera, as in Scalpellum, splits symmetrically between the several valves. In the branched filaments it is particularly difficult to understand their growth, for it is not possible, after examining them, to doubt that they continue to increase, and send off sub-branches, which it would appear probable, penetrate the shark’s flesh like roots. ”

“ The former division are distinguished by the possession of a swimming bladder, and by the nearly equal developement of the parts corresponding to the extremities—the pectoral and ventral fins. The pectoral fins, situated immediately behind the head, are present in all the osseous fishes, traces of them being found even where they might be supposed to be entirely wanting, as in the eel; they do not, however, attain any very considerable size, rarely equalling, and never surpassing, the magnitude of the ventral fins. ”

“ Before the appearance of a third visceral cleft in a part of the innermost layer of each protovertebræ (which may be called the splanchnic layer, from its being continuous with the mesoblast of the splanchnopleure) , opposite the bottom of the neural tube, some of the cells commence to become distinguishable from the rest, and to form a separate mass. This mass becomes much more distinct a little later, its cells being characterised by being spindle-shaped, and having an elongated nucleus which becomes deeply stained by reagents (Pl. 4, fig. 11, mp ́) . ”

“ This seems to show that the nervous plexus, if present, has its constituent fibres aggregated into trunks in the course of the nutriment tubes.
Thus far, then, I should be inclined to adopt the nerve-plexus theory. But lastly, we come to another [24] species with a very large bell and a very small polypite. Irritation of margin or radial tubes causes the animal to go into a violent spasm, but irritation of the general muscular layer only causes an ordinary locomotor contraction. ”

“ PECTEN ZELANDIÆ (Plate X.) .—Fig. 3 is a still smaller shell, and the most brilliantly coloured of our Pecten family. The valves are similar in shape, and covered with short spikes, It has only the one ear, or lug, at the hinge end, but sometimes a portion of the ear is found on the other side. This shell lives amongst rocks, or in sponges, or on the roots of kelp, in sheltered or fairly sheltered portions of open beaches. It is found attached to the rocks by a byssus, or beard. ”

“ The pectoralis major rises from the inner half of the clavicle, the front of the sternum, and the cartilages of the true ribs and its fibers converge to form a fan-shaped muscle, which is inserted by a flat tendon into the edge of the bicipital groove on the humerus. It draws the arm forward and inward and helps considerably in forced inspiration. The serratus magnus rises from the outer surface and upper border of the eight upper ribs and from an aponeurosis covering the upper intercostal spaces, and is inserted along the whole length of the posterior border of the scapula. ”

“ Pectoral Muscles.—The pectoral set of muscles varies but little in the family; flight power is seemingly not dependent upon size of either the pectoralis major or pectoralis minor. The data indicate that the insertion on the humerus, with consequent changes in the relative length of that bone, is more significant in type of flight and over-all flight power than is the actual size of the muscle mass. The deltoid muscle, for example, is smaller in Bombycilla than in members of the other two subfamilies. ”