CHAPTER XXV

THE PROTOCHORDATA

=The Chordate Animals.=--Referring to our metaphor of the tree with its twigs as used in the chapter on classification we find the fishes with the higher vertebrates as parts of a great branch from which the lower twigs have mostly perished. This great branch, phylum, or line of descent is known in zoology as _Chordata_, and the organisms associated with it or composing it are chordate animals.

The chordate animals are those which at some stage of life possess a notochord or primitive dorsal cartilage which divides the interior of the body into two cavities. The dorsal cavity contains the great nerve centers or spinal cord; the ventral cavity contains the heart and alimentary canal. In all other animals which possess a body cavity, there is no division by a notochord, and the ganglia of the nervous system if existing are placed on the ventral side or in a ring about the mouth.

=The Protochordates.=--Modern researches have shown that besides the ordinary back-boned animals certain other creatures easily to be mistaken for mollusks or worms and being chordate in structure must be regarded as offshoots from the vertebrate branch. These are degenerate allies, as is shown by the fact that their vertebrate traits are shown in their early or larval development and scarcely at all in their adult condition. As Dr. John Sterling Kingsley has well said: "Many of the species start in life with the promise of reaching a point high in the scale, but after a while they turn around and, as one might say, pursue a downward course, which results in an adult which displays but few resemblances to the other vertebrates." In the Tunicates or Ascidians (sea-squirts, sea-pears, and salpas), which constitute the class known as _Tunicata_ or _Urochordata_, there is no brain, the notochord is confined to the tail and is usually present only in the larval stage of the animal when it has the form of a tadpole. In later life the animal usually becomes quiescent, attached to some hard object, fixed or floating. It loses its form and has the appearance of a hollow, leathery sac, the body organs being developed in a tough tunic. There are numerous families of Tunicates and the species are found in nearly all seas. They suggest no resemblance to fishes and look like tough clams without shells. The internal cavity being usually filled with water it is squirted out through the two apertures when the animal is handled. The class _Enteropneusta_ (_Adelochorda_, or _Hemichordata_), includes the rather rare worm-like forms related to _Balanoglossus_. Bateson has shown that these animals possess a notochord which is developed in the anterior part of the body. They have no fins and before the mouth is a long proboscis. Gill-slits are found in the larval tunicate. In _Balanoglossus_ these persist through life as in the fishes.

The remaining chordate forms constitute the vertebrates proper, not worm-like nor mollusk-like, the notochord not disappearing with age, except as it gives way, by specialized segmentation to the complex structures of the vertebral column. These vertebrates, which are permanently aquatic, are known in a popular sense as fishes. The fish, in the broad sense, is a back-boned animal which retains the homologue of the back-bone throughout life, which does not develop jointed limbs, its locomotive members, if present, being developed as fins, and which breathes through life the air contained in water by means of gills. This definition excludes the Tunicates and Enteropneusta on the one hand and the Amphibia or Batrachia with the reptiles, birds, and mammals on the other. The Amphibia are much more closely related to certain fishes than the classes of fishes are to each other. Still for purposes of systematic study, the frogs and salamanders are left out of the domain of ichthyology, while the Tunicata and the Enteropneusta might well be included in it.

The known branchiferous or gill-bearing chordates living and extinct may be first divided into eight classes--the _Enteropneusta_, the _Tunicata_, the _Leptocardii_, or lancelets, the _Cyclostomi_, or lampreys, the _Elasmobranchii_, or sharks, the _Ostracophori_ the _Arthrodira_, and the _Teleostomi_, or true fishes. The first two groups, being very primitive and in no respect fish-like in appearance, are sometimes grouped together as _Protochordata_, the others with the higher Chordates constituting the _Vertebrata_.

=Other Terms used in Classification.=--The Leptocardii are sometimes called Acraniata (without skull), as distinguished from the higher groups, Craniota, in which the skull is developed. The _Leptocardii_, _Cyclostomi_, and _Ostracophori_ are sometimes called _Agnatha_ (without jaws) in contradistinction to the _Gnathostomi_ (jaw mouths), which include the sharks and true fishes with the higher vertebrates. The sharks and Teleostomes are sometimes brought together as _Pisces_, or fishes, as distinguished from other groups not true fishes. To the sharks and true fishes the collective name of _Lyrifera_ has been given, these fishes having the harp-shaped shoulder-girdle, its parts united below. The _Ostracophores_ and _Arthrodires_ agreeing in the bony coat of mail, and both groups now extinct and both of uncertain relationship, have been often united under the name of _Placoderms_, and these and many other fishes have been again erroneously confounded with the Ganoids. Again, the Teleostomi have been frequently divided into three classes--_Crossopterygii_, _Dipneusti_ or _Dipnoi_, and _Actinopterygii_. The latter may be again divided into _Ganoidei_ and _Teleostei_ and all sorts of ranks have been assigned to each of these groups. For our purposes a division into eight classes is most convenient, and lowest among these we may place the _Enteropneusta_.

=The Enteropneusta.=--Most simple, most worm-like, and perhaps most primitive of all the Chordates is the group of worm-shaped forms, forming the class of _Enteropneusta_. The class of _Enteropneusta_, also called _Adelochorda_ or _Hemichordata_, as here recognized, consists of a group of small marine animals allied to the genus _Balanoglossus_, or acorn-tongues (~balanos~, acorn; ~glôssa~, tongue). These are worm-like creatures with fragile bodies buried in the sand or mud, or living under rocks of the seashore and in shallow waters, where they lie coiled in a spiral, with little or no motion. From the surface of the body a mucous substance is secreted, holding together particles by which are formed tubes of sand. The animal has a peculiar odor like that of iodoform. At the front is a long muscular proboscis, very sensitive, capable of great extension and contraction, largely used in burrowing in the ground, and of a brilliant orange color in life. Behind this is a collar which overlaps the small neck and conceals the small mouth at the base of the proboscis. The gill-slits behind the collar are also more or less concealed by it.

The body, which is worm-like, extends often to the length of two or three feet. The gill-slits in the adult are arranged in regular pairs, there being upwards of fifty in number much like the gill-slits of the lancelet. As the animal grows older the slits become less conspicuous, their openings being reduced to small slit-like pores.

In the interior of the proboscis is a rod-like structure which arises as an outgrowth of the alimentary canal above the mouth. In development and structure this rod so resembles the notochord of the lancelet that it is regarded as a true notochord, though found in the anterior region only. From the presence of gill-slits and notochord and from the development and structure of the central nervous system _Balanoglossus_ was recognized by William Bateson, who studied an American species, _Dolichoglossus kowalevskii_, at Hampton Roads in Virginia in 1885, and at Beaufort in North Carolina, as a member of the Chordate series. Unlike the Tunicates it represents a primitively simple, not a degenerate, type. It seems to possess real affinities with the worms, or possibly, as some have thought, with the sea-urchins.

A peculiar little creature, known as _Tornaria_, was once considered to be the larva of a starfish. It is minute and transparent, floating on the surface of the sea. It has no visible resemblance to the adult _Balanoglossus_, but it has been reared in aquaria and shown to pass into the latter or into the related genus _Glossobalanus_. No such metamorphosis was found by Bateson in the more primitive genus _Dolichoglossus_, studied by him. This adult animal may be, indeed, a worm as it appears, but the presence of gill-slits, the existence of a rudimentary notochord, and the character of the central nervous system are distinctly fish-like and therefore vertebrate characters. With the Chordates, and not with the worms, this class, _Enteropneusta_ (~enteron~, intestine; ~pnein~, to breathe), must be placed if its characters have been rightly interpreted. It is possibly a descendant of the primitive creatures which marked the transition from the archaic worms, or possibly archaic Echinoderms, to the archaic Chordate type.

It is perhaps not absolutely certain that the notochord of _Balanoglossus_ and its allies is a true homologue of the notochord of the lancelet. There may be doubt even of the homologies of the gill-slits themselves. But the balance of evidence seems to throw _Balanoglossus_ on the fish side of the dividing line which separates the lower Chordates from the worms.

It may be noticed that Hubrecht regards the proboscis of various marine Nemertine worms as a real homologue of the notochord, and other writers have traced with more or less success other apparent or possible homologies between the Chordate and the Annelid series.

=Classification of Enteropneusta.=--Until recently the _Enteropneusta_ have been usually placed in a single family or even in a single genus. The recent researches of Professor J. W. Spengel of Giessen and of Professor William Emerson Ritter of the University of California, have shown clearly that the group is much larger than had been generally supposed, with numerous species in all the warm seas. In Spengel's recent paper, "Die Benennung der Enteropneusten-Gattungen," three families are recognized with nine genera and numerous species. At least seven species are now known from the Pacific Coast of North America.

=Family Harrimaniidæ.=--In _Harrimania maculosa_, lately described by Dr. Ritter from Alaska, the eggs are large, with much food yolk, and the process of development is probably, without _Tornaria_ stage. A second species of _Harrimania_ (_H. kupferi_) is now recognized from Norway and Greenland. This genus is the simplest in structure among all the Enteropneustans and may be regarded as the lowest of known Chordates, the most worm-like of back-boned animals.

In _Dolichoglossus kowalevskii_ the species studied by Bateson on the Virginia coast, the same simplicity of development occurs. This genus, with a third, _Stereobalanus_ (_canadensis_), constitutes in Spengel's system the family of _Harrimaniidæ_.

=Balanoglossidæ.=--The family _Glandicepitidæ_ contains the genera _Glandiceps_, _Spengelia_, and _Schizocardium_. In the _Balanoglossidæ_ (_Ptychoderidæ_ of Spengel) the eggs are very small and numerous, with little food yolk. The species in this family pass through the Tornaria stage above described, a condition strikingly like that of the larval starfish. This fact has given rise to the suggestion that the Enteropneusta have a real affinity with the Echinoderms.

The _Balanoglossidæ_ include the genera _Glossobalanus_, _Balanoglossus_, and _Ptychodera_, the latter the oldest known member of the group, its type, _Ptychodera flava_, having been described by Eschscholtz from the Pacific Coast in 1825, while _Balanoglossus clavigerus_ was found by Della Chiaje in 1829.

=Low Organization of Harrimaniidæ.=--Apparently the _Harrimaniidæ_, with simpler structure, more extensive notochord, and direct development, should be placed at the bottom as the most primitive of the Enteropneustan series. Dr. Willey, however, regards its characters as due to degeneration, and considers the more elaborate _Balanoglossidæ_ as nearest the primitive type. The case in this view would have something in common with that of the _Larvacea_, which seems to be the primitive Tunicates, but which may have been produced by the degeneration of more complex forms.