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.