Scuta

“ When surface markings exist on the scuta, they are very useful in defining species. The color is also a good character in some groups. Where one of these fails us the other very generally assists. The form of the last scutum is sometimes distinctive. The best characters, however, are those derived from the genital organs. The form, &c., of the antennæ and head often afford some clue.
Genus POLYDESMUS, Latreille. [24]
Meatus laterales in scutis 5—7—9—10—12—13—15—16—17—18—19—positus. ”

“ The head is broad, and has its anterior surface pubescent. The antennæ are rather long, filiform, pubescent, and strongly clavate. The first scutum is transversely oval. It is narrower than the head. The surface of the scuta is covered with minute flattened tubercles. On the more anterior these are quite distinct and arranged somewhat regularly in a fourfold series. On the more posterior they become less and less distinct, so that in some individuals these scuta have scarcely more than a rough appearance. ”

“ Scuta 8. Segments 16. Stomata of the scuta median. Feet and antenna multiarticulate.
In the Cermatiidæ the head is large, more or less truncate anteriorly, having its surface rough and uneven. The eyes are compound and very prominent. The palpi very elongate and spinous. The antennæ very long, and composed of a multitude of joints. The mandibles are elongate and distant. The scuta (eight in number) are rounded and deeply emarginate posteriorly, where they are furnished with a longitudinal, slit-like orifice or stoma. Their surface is more or less roughened with minute tubercles or spines. ”

“ Scuta oval, of varying proportions; the basal margin is generally narrow, and blends into the [Pg 101] carina-tergal margin; the internal basal rim generally is well developed, sometimes with, and sometimes without internal teeth beneath the umbones. In P. eburnea, and sometimes in P. crassa, there is a line of apparent fissure, and in P. fissa of actual disseverment, running from the umbo to the apex of each scutum, nearly in the line in which a ridge extends in Lepas: the primordial valves of the scuta in these three species, are seated at the basal angles of the lateral and larger segments. ”

“ Valves white, smooth, moderately thick, marked by the lines of growth. The occludent segments of the scuta, and nearly the whole of the terga, and the whole of the carina, enveloped in lemon-yellow membrane, tinged with orange, but the specimen had long been kept dry.
Scuta formed of two, apparently always separate, segments, closely united, so that externally their separation [Pg 110] is hardly visible, and does not allow of movement; the fissure thus formed runs almost in the line connecting the umbo and apex, (where in most species a ridge extends,) but a little on the carinal side of it. ”

“ The Scuta are triangular, with their umbones or centres of growth at the apex; the tergal margin, as seen from within, is either nearly straight or much hollowed out, accordingly as the scuta simply overlap the terga, or are received in a furrow. In some of the species there is a distinct pit for the adductor muscle, and in others this cannot be distinguished. ”

“ The basal portion of the mandibles on each side project, so that if a vertical section of the head through the middle were taken it would be an oval with its greater diameter transverse. The eyes are in strictly triangular patches, with remarkably straight sides. The scuta are so deeply canaliculate along their dorsal centre as to be almost divided into two parts. The dorsum is not perfectly rounded, but there is somewhat of a ridge or angle in the centre. Each scutum is furnished with a regular series of distant large puncta, or perhaps more properly, pores. ”

“ They are connected by a black band, which is much broader in the middle than at the ends. The first scutum is large, with very small lateral processes even in the female. There is on its anterior edge a very obscure light border, and a little posterior to this a black, more or less obsolete, band. The scuta are regularly, although rather lightly and distantly canaliculate over their whole surface. The last scutum is distinctly but minutely punctate, has its edge whitish, and is not at all mucronate. The anal scales are not at all pilose. ”

“ Even in the same species, there is often very considerable variation in the exact shape of the valves, more especially of the terga. The adductor muscle is always attached to a point not far from the middle of the scuta, and it generally has a pit for its attachment. In several genera, namely, Pæcilasma, Dichelaspis, Conchoderma, and Alepas, the scuta show a tendency to be bilobed or trilobed. ”

“ The colour of the shell varies from reddish-purple to nearly white; the radii are sometimes quite white; the walls are slightly ribbed. The scuta in the above specimens, externally were smooth; the adductor ridge was rather more distinct from the articular ridge than in any other variety; and the terga rather more plainly beaked. The tubular, up-curved, calcareous spines sometimes occur only on one side of the shell, and often only in the lower part. These spines are often coloured brighter than the walls. ”

“ The color of this species is a dark reddish chestnut, with the lateral laminæ of bright yellow, sometimes verging towards orange. The under surface of the body is yellowish. The scuta are wrinkled as in P. corrugatus, but not to so great an extent, at least in the individuals that have fallen under my observation. The head is of the same color as the body; its vertex furrow is strongly pronounced; its anterior margin yellowish and broadly emarginate. The antennæ are lightish chestnut, and distally very pilose, almost pubescent. The posterior scutum is triangular, with its apex truncate ”

“ There is, in all cases, the usual adductor scutorum muscle (with transverse striæ) , which is attached under the horny scuta, where such exist. The fact of the striæ-less muscles of the peduncle surrounding the whole capitulum, has been observed only in one other genus, namely Anelasma. In consequence of this structure, [Pg 158] the capitulum must possess considerable powers of contraction.
The antennæ of the larva in the Alepas cornuta and A. minuta have the sucking disc nearly circular, with the spines unusually plain on the distal as well as proximal margin. ”

“ The sutural edges of the alæ are generally distinctly crenated. The basis is often slightly cup-formed, and very plainly porose: its surface is marked by radiating ridges. The orifice of the shell is large, and elongated, especially in young specimens. The basal diameter of the largest specimen is .4 of an inch.
The Scuta have no trace of the two or three longitudinal furrows so conspicuous on these valves in B. bisulcatus, and which, in that species, run down from the apex of the valve; this fact showing that the furrows occur in quite young individuals. ”

“ The Scuta differ from those of B. psittacus only in the basi-tergal [Pg 210] corner not being so much rounded off, and consequently in the articular ridge, which is rather more reflexed, descending in proportion lower down the valve: the cavity at the basi-tergal corner is in proportion broader. The valves in the two species differ, also, but only in young specimens, in the occludent half being tinted, both externally and internally, purple, whereas in B. psittacus the whole valve, at all ages, is white. ”

“ Scuta, broad, feebly tinted with pink; exterior surface rough, with sharp hood-formed projections, arranged in straight lines radiating from the apex; an inflected portion of the valve along the tergal margin is not roughened. Internally (Pl. 3, fig. 1 d) , the articular ridge is broad and reflexed. An adductor ridge can hardly be said to exist, but a slight prominence borders the gentle hollow in which the lateral depressor muscle is attached. The basal margin, on its inner face, is slightly toothed. ”

“ Abdomen: Badly curled up; scuta entirely covered with short almost clavate hairs.
The naming of this species is based on the short "saw-like" hairs that are all over the body. They are not globular on the end, as the clavate hairs, but have "saw-like" edge.
Palps: Short and stout, coxa smooth, trochanter as usual, femur shorter than cephalothorax; pedicellate, inner margin almost straight at base, then suddenly concave to tip, outer margin evenly but not strongly convex; tibia broad, pedicellate, suddenly enlarging on inner side near base, outer margin evenly convex. ”

“ Abdomen: Twice as long as it is broad and divided into eleven distinct sutures. All of the scuta about the same size except the last one, which is a great deal shorter and broader than the rest. Each scutum is provided with two strong, spiny hairs on the outer edge.
The whole body is heavily granulated, the cephalothorax having knob-like protuberances all along the edges.
Pedipalps: Larger than the whole animal. Coxa, smooth; trochanter with large protuberance ending in a heavy spine on the outer edge. Femur longer than cephalothorax, pedicellate. ”

“ I presume that they are homologous with the carinal latera in Scalpellum. Each successive layer of shell forming the valves is thick, and extends over nearly the whole inner surface; hence the carina and terga, and to a certain extent the scuta, either actually do project freely much beyond the sack, or would have done so, had not their upper ends been removed; for the upper and old layers of shell, in most of the species, either scale off or disintegrate and wear away. ”

“ In the depressed much corroded specimens, the terga (fig. 2 f) , like the scuta, [Pg 352] are broader and shorter than in the commoner variety; and the spur more especially is broader. The scutal margin is much more widely inflected, and the articular ridge much more prominent; consequently the articular furrow is much deeper.
Structure of the parietes and radii.—The orange or yolk-of-egg colour, which is so conspicuous a character in the present species, is due to a layer of shell between the inner and outer lamina, and is exposed only by the corrosion of the latter. ”

“ Sterna of the posterior subsegments almost absent, of the anterior moderate, both closely conjoined with the scuta. Scuta not furnished with lateral lamina.
The head in this family is moderately large, and generally has the organs of special sense well developed. The eyes are present in all our American species, arranged in variously shaped patches, near to the base of the antennæ. The latter are sometimes long and filiform; but in the genus Spirobolus they are generally short and massive. The body is cylindrical and composed of very numerous segments, which are more or less imbricated. ”

“ This faculty of reproduction is doubtless designed to enable the creature to escape from its assailants: the detaching of the limb is evidently its own act; and it is observable, that when reproduced, the tail generally exhibits some variation from its previous form, the diverging spines being absent, the new portion covered with small square uniform scales placed in a cross series, and the scuta below being seldom so distinct as in the original member. ”

“ The antennæ are formed according to the type of the Diplopoda, with seven joints. The body is composed of numerous segments. The scuta may be prolonged laterally into laminæ, but afford protection only to the back and flanks; the central part of the abdomen is not covered by a firm, resistant shell. The feet are hidden beneath the broad body. The male genitalia are placed in the seventh segment. In the classification of the families and genera I have followed Mr. Brandt, [26] his arrangement, as far as the two species I have seen enable me to judge, being founded on natural characters. ”

“ Ferruginous; feet yellow; cephalic segment polished, with its posterior margin elevated; antenna elongate, pilose; dental lamina indistinct, each armed with two widely separated denticules; pairs of ocelli 17; alternate larger scuta polished, scarcely roughened, with their posterior margins almost straight; alternate minor scuta, with their posterior margins straight and their external angles produced; excavations on the coxa small, scarcely oval. ”