Comprehensive DescriptionRead full entry
“Pista mirabilis, n. sp. (Pl. LI. figs. 2; Pl. XXVIIA. fig. 34; Pl. XXXVIIIA. fig. 2).
Habitat.—Procured at Station 320 (off the mouth of the Rio de la Plata), February 14, 1876; lat. 37° 17' S., long. 53° 52' W.; depth, 600 fathoms ; bottom temperature 37°·2, surface temperature 67°·5 ; sea-bottom, green sand.
A species inhabiting remarkable leathery tubes. Its length is about 58 mm., and its diameter anteriorly is rather more than 2 mm.
On removal from the tube the body presents a dull flesh colour or pale madder-brown hue, and the greatly developed cephalic tentacles are of the same tint. On comparing the anterior region with that in Pista cristata certain differences are apparent ; thus the foliaceous lamella of the third segment, instead of forming an elongated process stretching from the base of the second branchia nearly to the ventral scute, and regularly diminishing from above downward as in Pista cristata, forms a prominent tongue-shaped process behind the single branchial column, and directed forward. Moreover, this process runs into the fold immediately in front of the first hook-row, a narrow ridge devoid of the lamella, observed in Pista cristata, intervening between it and the great suboral plait. The single pair of branchiæ which are inserted in front of the tongue-shaped lamella, would appear to correspond with the first pair in the British form, and therefore belong to the second segment. The pedicle of the branchia is long, stout, and transversely corrugated, and the terminal tuft of branches is comparatively small. No tendency to a whorled arrangement exists in the tuft, but the main trunk divides into three, and then each division splits into various processes. The ultimate twigs, which are thicker than those of Pista cristata, are dichotomously divided. The whole somewhat resembles the branchia of a Terebella, being sparsely branched, and situated at the summit of a long pedicle.
The bristles are much longer and more tapered than those of Eupista darwini or other allied form, a considerable portion beyond the slightly developed lateral wings being extremely attenuated. A greater amount of the whitish glandular tissue surrounds the first four hook-rows than in Pista cristata. The dorsal processes that occur behind the third, fourth, and fifth, however, are less developed than in the latter form.
The hook-pads (uncinigerous eminences) are much shorter than in Pista cristata. The hooks (Pl. XXVIIA. fig. 34) differ from all the previous types in the comparative flatness of the crown, which in profile shows about three teeth, in the proportionally small space below the great fang, and in the boldly convex nature of the anterior inferior prominence, which is so developed as to give great massiveness to the base of the hook. The posterior or dorsal outline, moreover, is characteristic, being only slightly indented about the middle, and devoid of the usual projection at the base of the long process. The latter is comparatively slender, and issues only from the posterior angle, instead of having the usual wide connection with the adjoining base. The latter presents a firmer condition than is common, and is marked by minute crenations. The characters of the entire organ are constant and easily defined.
The intestine contains a little sand, in which are a few Diatoms and fragments of sponge-spicules. The Gregarinæ in the canal are well formed, the larger presenting distinct longitudinal bands, apparently of a contractile nature.
The dark greyish or somewhat olive tubes (Pl. LI. fig. 2) are tolerably firm, rounded, chitinous structures, tapering from the anterior to the posterior extremity, and armed all over with long spinous processes. The majority of the tubes seem to have been free, but others have been immersed in sponges, a position which has favoured the preservation of the long external spines. In intimate structure the wall of the tube is marked by close wrinkles, which are so fine as almost to be linear, a feature partly due to its composition, for it consists of a vast number of needle-like glassy spicules of sponges, held together by secretion and mud. This composition gives a gritty feeling on touching the tube, while it more readily enables it to retain its circular form. Arranged somewhat alternately all over the tube, though more sparsely at the wide or anterior end, and gradually disappearing at the narrow one, are a series of spinous processes, which give the tube a characteristic appearance. They project outward in some instances a considerable length, equalling indeed several times the diameter of the tube. These appendages are lighter in colour than the latter, but are composed of similar materials, viz., sponge-spicules, secretion, and sand-grains, the latter occurring in greater quantity than in the tube proper. The proportionate diminution of the mud probably renders the spinous processes pale. The most perfect spines occur amongst the masses of sponge encrusting certain tubes, and are fully 16 mm. in length, of a pale straw colour, and almost entirely composed of secretion and sponge-spicules. It is interesting to notice how neatly the sponge-spicules are ranged longitudinally in these processes, a considerable amount of design being apparent in every instance. From the great number as well as the length of the spines amongst the encrusting sponge, it would appear that a special advantage had been gained, other than is observable in the protective function of the sponge, or that special efforts had been made under the circumstances. The spines have a broad base of attachment, and then are slightly tapered upward to the point, the spicules at the tip being drawn together to form a termination. Besides the spicules, numerous twigs of Polyzoa are attached to the tubes.
The tubes are all simple, the only apparent branching being due to the attachment of a tube belonging to another species, or a smaller one of the same form. The length varies, the longer examples reaching 150 to 160 mm., with a diameter of about 4 mm. Some present a soft flexible prolongation at the posterior extremity.
Microscopically the wall of the tube shows a vast series of sponge-spicules, often laid in close parallel rows, numerous sand-grains, and fine particles of mud. A few Diatoms, bristles of Annelids, and small Foraminifera also are present.
In the interior of one of the tubes are many ova, arranged in a somewhat linear manner, but such may have been due to rupture of the body of the parent and not to any special nidamental disposition. The Annelid was distended with ova in various stages of development.
The sections of the anterior third of the body-wall of this species present a characteristic appearance, since they are more definite and firm than usual in the group (Pl. XXXVIIA. fig. 2). The hypoderm forms a thin coat dorsally, but assumes greater bulk toward the infero-lateral regions. In the ventral median line it constitutes a thick external envelope to the nerve-area. Moreover, in the preparations it seems to have an intimate relation to a large mass of similar structure and of a somewhat foliate aspect, extending between the oblique and ventral muscles, and superiorly almost touching the alimentary canal. This constitutes a great glandular mass, with whitish opaque regions here and there, which must have a special function, probably of secretion. The large and rounded nerve-cords are situated in the outer portion of the median hypoderm, their inner (upper) boundary being the circular muscular coat, while a belt of hypodermic fibres is placed externally. A small neural canal lies between them. The circular muscular coat is of more than average strength. A thin band of longitudinal fibres lies on the inner surface of the latter coat over the nerve-area. Moreover, in what appear to be the interganglionic regions, the same changes in the relationship of the nerve-cords to the circular coat occur as in Pista abyssicola. The hiatus between the ventral longitudinal muscles is lessened, the cords pass to the inner surface of the circular coat, having internally (superiorly) only a basement-band, the thin stratum of longitudinal fibres, and the foliate glandular masses. The hypoderm outside the circular coat, again, is at once firmer and narrower. The oblique muscles are well formed and powerful, and are inserted at the outer borders of the wide median hiatus, which is about thrice the transverse diameter of the nerve-area. The longitudinal ventral muscles are firm, wedge-shaped masses bounded internally by the oblique, and externally by the circular muscular coat. The longitudinal dorsal form extended plates of nearly uniform diameter, and separated superiorly by a rather wide hiatus. From the latter issues the broad suspensorial band of the alimentary canal. The latter is firm and finely striated from the closely arranged glands. The œsophageal region has an external investment, showing many longitudinal fibres, a thick circular muscular layer, and a symmetrically folded glandular lining. The glands on the edges of the opposing folds have such a disposition that they resemble ears of wheat. The perivisceral chamber also contains the ovaries with ova at various stages.”