These remarks lead me to the consideration of one or two very remarkable animals that have recently been brought to light.
In that strange assembly of animals which, for want of a better word, the authorities call the Vermes, there are three groups whose relations to one another and to the other groups of Vermes have been and still remain a puzzle to naturalists.
These three groups are the Gephyrea, the Polyzoa, and the Brachiopoda. In external form they are as different from one another as possible.
The Gephyrea are solitary worm-like forms burrowing in the sand or perforating rocks; the Polyzoa are minute creatures that frequently build up by budding large colonies which assume in some cases dendritic forms like corals, and the Brachiopoda are protected by thick bivalve shells simulating in a striking manner the shells of the Lamellibranchiate mollusca.
But external form is not the only character that can be relied upon for purposes of classification. The general and minute anatomy, together with the story of the development of these animals, teach us that they are in some way closely related.
It is not within the scope of this book to enter into the discussion of what these relations are; suffice it to say that the controversy has within recent years to a great extent turned upon the position in our classification of three interesting genera. These are Phoronis, Rhabdopleura and Cephalodiscus.
Phoronis occurs only in shallow water, Rhabdopleura has been found in water from 40 to 200 fathoms deep off the Shetlands and on the Norwegian coasts, while Cephalodiscus was discovered by the ‘Challenger’ at a depth of 245 fathoms off Magellan Straits.
Rhabdopleura forms colonies consisting of branched tubes growing upon the tests of Ascidians, on sea-weeds, corals, or other objects fixed to the sea bottom. In the open, free extremity of each of the branches may be found the polypide attached to a filament or stalk which connects it with the other polypides of the colony (fig. [12]).
Fig. 12.—A portion of a colony of Rhabdopleura normani. (After Lankester.)