CHORDATA AND VERTEBRATA—HEMICHORDATA—ENTEROPNEUSTA—EXTERNAL CHARACTERS AND HABITS—STRUCTURE—GENERA—DEVELOPMENT—PTEROBRANCHIA—CEPHALODISCUS AND RHABDOPLEURA—PHORONIDEA—PHORONIS AND ACTINOTROCHA—AFFINITIES OF THE HEMICHORDATA.
The Hemichordata, a marine group which includes the worm-like Balanoglossus, owe much of their interest to the fact that they are believed by many zoologists to be related to the lower Vertebrates. This view is one of a number of mutually exclusive hypotheses, which seek to derive Vertebrate animals from various Invertebrate ancestors. It is supported by many striking resemblances between Balanoglossus and the lowest forms which are by common consent regarded as belonging to the Vertebrate alliance; but it must be distinctly understood that Balanoglossus is at most the much modified modern representative of extinct forms which were also the ancestors of Vertebrates.
The axis of the backbone of all Vertebrates is formed by an elastic rod known as the "notochord" (Figs. 72, 115), which lasts throughout life in some of the lowest forms, but in the higher forms appears only in the embryo. The universal occurrence of this structure has been regarded as the most important characteristic of the Vertebrata and their allies, which are accordingly grouped together in the Phylum CHORDATA. The members of this Phylum are further distinguished from other animals by several important features. Of these one of the most important appears to be the existence of lateral outgrowths of the pharynx, which unite with the skin of the neck and form a series of perforations leading to the exterior. These structures are the gill-slits, and in the Fishes their walls give rise to vascular folds or gills. With the assumption of a terrestrial life, the higher Vertebrates lost their gills as functional organs, respiration being then performed by entirely different organs, the lungs. But even in these cases, the gill-slits appear in the embryo; and remains of one pair can usually be recognised in the adult state of even the highest Vertebrates. Another fundamental characteristic of the Chordata is given by the central nervous system, which lies entirely above the alimentary canal, just dorsal to the notochord. Not only does this position of the nerve-centres distinguish the Chordata from Invertebrates, but a further point of difference is found in the development. While in Invertebrates the ventral nerve-cord is formed as a thickening of the ectoderm or outermost layer of the embryo, in the Chordata the nervous system is usually formed as a longitudinal groove running medianly along the back of the embryo. This groove closes to form a tube of nervous matter, the cavity of which always persists throughout life as the "central canal" of the spinal chord and its anterior prolongation which constitutes the "ventricles" of the brain.
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 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.
Fig. 1.—Forms of Balanoglossus. A, Balanoglossus clavigerus, Eschsch., Naples, × ½; B, Glandiceps hacksi, Mar. (incomplete), Japan, × 1; C, Schizocardium brasiliense, Speng., Rio de Janeiro, × 1; D, Dolichoglossus kowalevskii, A.Ag., Chesapeake Bay, × 1. a, Anus; ab, abdominal and caudal regions; b, branchial region; c, collar; g, genital region; g.p, gill-pore; g.w, genital wing; h, hepatic region; m, position of mouth; p, proboscis; t, trunk. (A, B, and C from Spengel; D from Bateson.)