Balanoglossus is not without relations, some of which have been recently discovered, while others have been known for some time, although their affinities were not at first recognised. Among these the most remarkable are sessile forms which have received the names respectively of _Cephalodiscus_ and _Rhabdopleura_. Both produce buds and form a colony, and in both a notochord has been distinguished. The former was procured from the Straits of Magellan, while the latter makes its dwelling-place in a nearer region, having been found off the Shetland Islands, and off the Lofoden Islands. Cephalodiscus, which is a very curious creature, receives its name from a disc placed at the head end. The use of this structure is believed to be as follows. The units of the colony live inside a common system of tubes, which they secrete; each unit, when adult, is independent, and can move about inside the tubes; the disc is used as a means of attachment to successive spots of the tube-wall, as the animal wanders from place to place. Above the disc are twelve plume-like tentacles covered with cilia, which create a current in the water surrounding the head, and wash food particles into the mouth. That these creatures are but distant relations of the true vertebrates is a fact expressed by the names under which they are grouped in classification. Those forms which we have just described have received the name of Hemichordata--that is to say, Chordata which have but half a notochord, since the notochord is very restricted in extent; while the Ascidians are grouped under the name of Urochordata, or Chordata which only possess a notochord in the tail. The name of Adelochorda, "with an obscure chord," is sometimes applied to the Hemichordata. TABLE SHOWING THE CLASSIFICATION OF THE CHORDATA { HEMICHORDATA, BALANOGLOSSUS, &c. =CHORDATA.= { UROCHORDATA, THE ASCIDIANS. { VERTEBRATA, THE BACK-BONED ANIMALS. Let us return now to the Vertebrate. A character common to all the groups of the Vertebrata is the possession of teeth. Readers of the previous volumes of this series will recollect that, even among birds, instances of the possession of teeth may be found among fossil forms, although they are absent in the birds of the present day. In all the other divisions of the Vertebrata, the presence of teeth is the rule, their absence an exception so rare that we may easily note the chief instances of it. Among Amphibia, there are Toads that have no teeth; among Reptiles, the Tortoises and Turtles have none; among Mammals, teeth are wanting in _Echidna_, the Spiny Ant-eater; and in the Ant-eaters and the Whalebone Whales they are absent in the adult, although present in early embryonic life.