. The fauna of the deep sea . eans sufficiently well understood. It is one ofthose forms that, without being, strictly speaking,a connecting link between large and well-knowngroups of animals, indicates to us some of the linesof evolution that these groups may liave passedthrough; and, in so far as it does this, it has itsvalue and importance. Cephalodiscus, though related to Rhabdopleura inthe presence of a structure corresponding to thearms, and a broad epistome, seems to be more closelyconnected with such a form as Balanoglossus in thepresence of a single pair of gill-slits, a smallrudiment
. The fauna of the deep sea . eans sufficiently well understood. It is one ofthose forms that, without being, strictly speaking,a connecting link between large and well-knowngroups of animals, indicates to us some of the linesof evolution that these groups may liave passedthrough; and, in so far as it does this, it has itsvalue and importance. Cephalodiscus, though related to Rhabdopleura inthe presence of a structure corresponding to thearms, and a broad epistome, seems to be more closelyconnected with such a form as Balanoglossus in thepresence of a single pair of gill-slits, a smallrudimentary notochord and the position of the centralnervous system. Whatever position these genera may ultimatelyoccupy in our systems of classification, there can belittle doubt that much valuable information will beobtained by a farther study of their structure and • A rudimentarynotocLord projecting forward from the buccalcavity into the epistome has quite recently been discovered inRhabdopleura. 9 114 THE FAUNA OF THE DEEP SEA. Fig. 13.—a single polypide of RJmhdoplenra normani. M,mouth ; b, epistome ; s, poljpide stalk. (After Lankester.) VERMES AND MOLLUSCA OF THE DEEP SEA 115 development—information that will probably shedmuch light on the relationships to one another of themany groups of Vermes. Their occurrence in waterof moderate depths only indicates perhaps that theyare gradually being crowded out from the morefavourable localities of shallow water, and are tendingtowards extinction on the one hand, or a deep-seahabitat on the other. The Brachiopoda need not detain us long. Somespecies are capable of existing at a great variety ofdepths without any observable modification of shapeor characters. Thus Terehratulina caput serjpentishas the extraordinary bathymetrical distribution of0-1,180 fathoms, and Terehratula vitrea 5-1,456fathoms. Atretia is the only genus peculiar to deepwater. It is a noteworthy fact in connection withthis order that the two genera, Lingida and
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