The scales of Fishes were at one time proposed as a basis of classification: large groups being characterized respectively by the possession of plain rounded scales (cycloids), scales fringed at the posterior end (ctenoid, or comb-like); placoid scales, consisting of bony plates, and ganoid scales, large plates covered with shiny enamel. These distinctions, however, were not found useful as a guide in classification. The diagram shows the elaborate scales of the common sole.
Fig. 42.—Scales of the Common Sole, highly magnified.
Let us now consider some other creatures that resemble vertebrates in some ways, and help to form the group of Chordata. Balanoglossus is one of them, the Acorn-tongue Animal. This odd name is given to it on account of a structure which is called (like the elephant's trunk) a Proboscis; this may be compared with a tongue, so far as its use goes, for it is thrust out to catch prey and again drawn in. It is oval in shape, and therefore fancifully compared to an acorn. It is highly sensitive, being richly supplied with nerves. The creature is to all intents and purposes a kind of worm; and, like many of the higher worms, it has a larva with bands of cilia. This larva, which is better represented in some species than in others, was originally described under the name of Tornaeria. It is considered to resemble, in some degree, the larva of Echinoderms; on this hint, some zoologists have sought to establish a connection between Vertebrates and Echinoderms, and have been able to find other points of comparison besides the one named. It remains to be seen whether this suggestion will lead to further results. It may be added that the larva of Balanoglossus has also been compared with that of Phoronis ([p. 122]), thus assuming a relationship with the Polyzoa, and through them with the Brachiopoda. It appears, therefore, that the subject of the possible relationships of the Vertebrata is one of the greatest complexity. The last named theory, however, has been adversely criticised by very high authority.
We have not, however, explained yet what is the claim of Balanoglossus to be grouped with the Chordata. This consists in the fact that a certain part associated with the interior of the proboscis has been identified, from its structure, mode of origin, and relations with the nerves, as a notochord. Balanoglossus also agrees with the true vertebrates in possessing successive pairs of perforating gills (see [p. 142]), which are especially noticeable in the young animal. The presence of this feature is important, in view of the fact that some authorities have sought to throw doubt on the genuineness of the notochord of Balanoglossus.
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.