Each polypide is provided with a single pair of large pinnate arms, resembling the arms of a Brachiopod, and a broad muscular epistome by means of which it is able to creep up or down the tube.
The affinities of this interesting creature are by no means sufficiently well understood. It is one of those forms that, without being, strictly speaking, a connecting link between large and well-known groups of animals, indicates to us some of the lines of evolution that these groups may have passed through; and, in so far as it does this, it has its value and importance.
Cephalodiscus, though related to Rhabdopleura in the presence of a structure corresponding to the arms, and a broad epistome, seems to be more closely connected with such a form as Balanoglossus in the presence of a single pair of gill-slits, a small rudimentary notochord and the position of the central nervous system.[[2]]
[2]. A rudimentary notochord projecting forward from the buccal cavity into the epistome has quite recently been discovered in Rhabdopleura.
Fig. 13.—A single polypide of Rhabdopleura normani. M, mouth; B, epistome; S, polypide stalk. (After Lankester.)
Whatever position these genera may ultimately occupy in our systems of classification, there can be little doubt that much valuable information will be obtained by a further study of their structure and development—information that will probably shed much light on the relationships to one another of the many groups of Vermes. Their occurrence in water of moderate depths only indicates perhaps that they are gradually being crowded out from the more favourable localities of shallow water, and are tending towards extinction on the one hand, or a deep-sea habitat on the other.
The Brachiopoda need not detain us long. Some species are capable of existing at a great variety of depths without any observable modification of shape or characters. Thus Terebratulina caput serpentis has the extraordinary bathymetrical distribution of 0–1,180 fathoms, and Terebratula vitrea 5–1,456 fathoms. Atretia is the only genus peculiar to deep water. It is a noteworthy fact in connection with this order that the two genera, Lingula and Glottidia, which compose the sub-order Ecardines, are both confined to shallow water. Now the Ecardines are anatomically, at any rate, the most primitive of the Brachiopoda, and Lingula has the most ancient geological history of any living genus of the animal kingdom, shells almost identical with those of the living species being found abundantly in the Cambrian strata. Why it is that Lingula has been able to maintain itself almost unchanged through all the countless generations that have elapsed since Cambrian times, and can now flourish amid the desperate struggle for existence in the shallow waters of the tropics, while its companions, the corals, mollusks, arthropods, &c., have changed or passed away, is one of those problems in natural history that seem to us impossible of solution. The time may come when we shall be able to appreciate better than we do now the complicated relations between animals and their environment, and then perhaps the peculiar fitness of Lingula will be made manifest; but at present we can but mention the fact as a fact, and leave the solution of the problem to the future.
The order Gephyrea is probably another very ancient group of animals, although in the absence of any hard calcareous, siliceous, or horny skeleton the geological record can give us no confirmation of their antiquity. As with the Brachiopods so with the Gephyrea, some of the species have a very wide bathymetrical distribution. Sipunculus nudus, for example, the commonest and best known of all the Gephyrea, extends from quite shallow water to a depth of over 1,500 fathoms. As Selenka has pointed out, it is those Gephyreans that live in holes in stones, or in shells such as Phascolion and Phascolosoma, that are more frequently found at the greater depths; but, apart from this, there are no characters that exclusively belong to the abysmal Gephyrea or are more frequently found in them than in the shallow-water forms. Nor are there any genera, at present brought to light, that are confined to those regions of the sea.
The group of the Annelida is not very well represented in the deep-sea fauna. The genera Serpula and Terebella have been found very widely distributed over the earth, at all depths from the shore to the abyss, but there do not seem to be many genera that are confined to deep water. In some cases, where there is a scarcity of lime in the water, the thin protecting tubes of the sedentary forms are strengthened by the adhesion of foreign particles, such as sponge spicules and arenaceous foraminifera, but in others, the tubes are formed of successive layers of a transparent quill-like substance (Nothria Willemoesii) which is frequently armed with spiny projections.