Although the animals which are considered in this chapter are not admitted by all zoologists to be related to the Vertebrates, there can be no question that their respiratory organs closely resemble typical gill-slits. Since, moreover, they possess structures which can be regarded, with a fair amount of probability, as agreeing in essential respects with the notochord and the tubular dorsal nervous system of Vertebrates, it appears justifiable to include them in the Chordata, which are then subdivided into (1) HEMICHORDATA, in which a "notochord" occurs in the anterior end of the body only; (2) UROCHORDATA (Tunicata or Ascidians), in which the notochord is restricted to the tail; (3) CEPHALOCHORDATA (Amphioxus), in which the notochord extends the entire length of the body _and of the head_; (4) CRANIATA, in which a brain is developed as an enlargement of the central nervous system, the notochord does not extend farther forward than the middle of the brain, and a vertebral column is present. These last are thus usually known as Vertebrata, although in distinguishing an "Invertebrate" from a "Vertebrate" it is more {5}logical to regard all Chordata as Vertebrates, since the Invertebrata are in no sense a natural group with common characteristics, their union under one name merely implying that they have no close affinity to the Vertebrates. It is often convenient in practice to divide animals into Vertebrates and Invertebrates, but from a zoological point of view a division of the animal kingdom into Molluscs and Non-Molluscs would have as much or as little significance. The sub-phylum Hemichordata[1] consists of the Orders:—(I.) ENTEROPNEUSTA,[2] including Balanoglossus (Fig. 1); (II.) PTEROBRANCHIA,[3] represented by the genera _Cephalodiscus_ (Fig. 9) and _Rhabdopleura_ (Fig. 12). To these should possibly be added (III.) PHORONIDEA, for the reception of _Phoronis_ (Fig. 13). ORDER I. ENTEROPNEUSTA. _Worm-like Hemichordata, with numerous gill-slits, a straight intestine, and a terminal anus. Proboscis separated by a narrow stalk from the simple ring-shaped collar, which is succeeded by an elongated trunk_. The structure of Balanoglossus, formerly the sole genus belonging to this Order, but now divided[4] into the genera _Ptychodera_, _Balanoglossus_, _Glossobalanus_, _Glandiceps_, _Spengelia_, _Schizocardium_, _Harrimania_, _Dolichoglossus_, and _Stereobalanus_, has of recent years formed the subject of elaborate investigations by Spengel,[5] Bateson,[6] and Willey.[7] More than thirty species are known, ranging in length from 25 mm.[8] (_Pt. bahamensis_) to 2500 mm. (_B. gigas_), and for the most part inhabiting shallow water; _Glossobalanus sarniensis_ occurring between tide-marks in the Channel Islands. _Glandiceps talaboti_ has, however, been dredged near Marseilles from as much as 190 fathoms, while _G. abyssicola_ was found by the "Challenger" at a depth of 2500 fathoms, off the West Coast of Africa.