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“Hyalincæcia tubicola, O. F. Muller, var. (Pl. XL. fig. 1).
Habitat.—Trawled in great numbers at Station 320 (off the coast of Buenos Ayres, South America, to the south of the estuary 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. It is stated on the label that the specimens came from hard ground, but this is unusual.
This large Annelid inhabits a tube about the size of a goose-quill, but much resembles the ordinary form in appearance, both in regard to animal and tube. The latter measures from 200 to 220 mm. in length, with a diameter at the wide end of 8 mm., and at the narrow extremity of 4 mm.
The head agrees with that of the typical form except in size and in the variable proportions of the organs. The eyes are absent. The dental armature (Figs. 88, 89) differs from the description of Ehlers in having fourteen teeth on the right great dental plate and seventeen on the left, being one more on each side, but the difference in size is striking. Moreover, the number of teeth is variable, and occasionally it corresponds with that in the typical examples.
The branchiæ in the British examples of Hyalinœcia tubicola commence on the twenty-fourth or twenty-fifth foot, while in this variety they begin on the twenty-fifth or twenty-sixth, occasionally on the twenty-seventh. The organs are proportionally larger, but of similar structure.
The bristles of the first foot are simple and smooth. The wide end of the tube has thin walls. At the narrow end are several diaphragms which are fixed to the sides.
In the anterior region the general aspect in transverse section corresponds with that in Hyalinœcia tubicola, though there are certain minor distinctions which, however, may be due to the greater size and development of all the parts. Thus the nerve-area, instead of resting for the most part freely on the commissure of the oblique muscles, is enveloped by a dense series of fibres, chiefly vertical and superior oblique, but also of fibres from the inferior border of the alimentary canal, which enclose a large blood-vessel. As in most large forms the nerve-area is proportionally smaller and more flattened than in the British species (the size of which is much less), but the neural canal occupies a similar position. The circular muscular coat, the hypoderm, and the cuticle have about the same bulk. A large blood-vessel occupies the intermuscular area dorsally, where two are conspicuous in the common form, and the great vascularity of the muscular and other tissues within the hypoderm is evident. Posteriorly the chief changes consist in the flattening of both dorsal and ventral muscles in accordance with the shape of the body, and the appearance of the reproductive elements at the bases of the feet. The nerve-area is much widened, but the cords are flattened.”